universidade de são paulo - usp · university of são paulo “luiz de queiroz” college of...

University of São Paulo “Luiz de Queiroz” College of Agriculture

Energy supplementation for beef steers grazing tropical grass (Brachiaria brizantha, cv Marandu) managed under rotational

system with different initial sward heights

João Ricardo Rebouças Dórea

Thesis presented to obtain the degree of Doctor in Science. Area: Animal Science and Pastures

Piracicaba 2014

João Ricardo Rebouças Dórea Agronomist

Energy supplementation for beef steers grazing tropical grass (Brachiaria brizantha, cv Marandu) managed under rotational system with different initial

sward heights versão revisada de acordo com a resolução CoPGr 6018 de 2011

Advisor: Prof. Dr. FLÁVIO AUGUSTO PORTELA SANTOS

Thesis presented to obtain the degree of Doctor in Science. Area: Animal Science and Pastures

Piracicaba 2014

Dados Internacionais de Catalogação na Publicação

DIVISÃO DE BIBLIOTECA - DIBD/ESALQ/USP

Dórea, João Ricardo Rebouças Energy supplementation for beef steers grazing tropical grass (Brachiaria

brizantha, cv Marandu) managed under rotational system with different initial sward heights / João Ricardo Rebouças Dórea. - - versão revisada de acordo com a resolução CoPGr 6018 de 2011. - - Piracicaba, 2014. 121 p: il.

Tese (Doutorado) - - Escola Superior de Agricultura “Luiz de Queiroz”, 2014.

1. Energia 2. Forragem tropical 3. Gado de corte 4. Manejo da pastagem 5. Suplementação I. Título

CDD 636.2084 D695e

“Permitida a cópia total ou parcial deste documento, desde que citada a fonte -O autor”

3

Offer

To my son and my wife,

Pedro Dórea and Renally Dórea

Whose sacrifices, which were realized by our loss of precious time together,

were for me the most painful of all.

Thank you for everything, I love you!!

To my parents and my siblings,

Tadeu Dórea, Márcia Rebouças, Ana Rebouças Dórea and

Antonio Dórea

For the dedication, understanding and above all, for their support. In the

most difficult times, they always helped me with valuable advices.

I love you so much guys!!!

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ACKNOWLEDGMENTS

To GOD for keep me with health and peace to work and do all my activities during my

Doctorate Program.

To Escola Superior de Agricultura “Luiz de Queiroz” (ESALQ/USP), Universidade de

São Paulo (USP) and especially to “Departamento de Zootecnia” for providing all

intellectual and material support necessary for the proper development of this work.

I would like to thank Fundação de Amparo a Pesquisa do Estado de São Paulo

(FAPESP) to finance me during my entire doctorate program, including my

experience at University of Wisconsin-WI (USA).

I would like to express my deepest gratitude to my advisor, Dr. Flávio Santos, for his

excellent guidance, confidence, patience, and providing me with an excellent

atmosphere for doing research. I would like to thank Dr. Sila da Silva, who teaches

me a lot about grazing management, what give me a considerable differential. I also

would like to thank Dr. Luiz Gustavo Pereira, for guiding my research for the past

several years and helping me to develop my background in in vitro rumen

degradation techniques.

I would like to thank Dr. Armentano, who was willing to advise me during the time that

I stayed in University of Wisconsin, being fundamental to show me a completely

different way to think about research.

I would like to thank Luiz Roberto (Conçolo), Vinícius Gouvea (Goiano) and Murilo

Garret, as a good friend, was always willing to help and give their best suggestions. It

would have been an impossible time without them.

I would also like to thank my big friend Marina Danés (Uruk), who gave an

immeasurable support for me and my family during our period in Madison-WI. We

lived happy and hard moments, but I am sure unforgettable moments! Thanks for the

confidence to recommend me to work at UW.

Many thanks for all my friends that helped me: Prof. Carla Bittar, Acreano, Paraíba,

Titanic, Jonas, Ferndana, Humberto, Cris, Camila, K-Bomba, Deu-c-ja, 100-kina, and

other friends that help me. Also, I would like to thank Cesar, who help me a lot in the

Lab analysis.

I would like to thank all my family especially Luciano Dórea, Olívia Rebouças, Luiz

Hage, Alba Lúcia, Waldemar da Fonseca, Ricardo Amaral, George Sodré, Maurício

Dantas and everyone who always wished me success.

6

I would like to thank my parents-in-law Mr. Chagas and Mrs. Iaponan that always

gave me love as their son and supported me during my PhD program.

I would like to thank Bruno and all workers of Suplementar Company to show me the

other side of Livestock Science. Where are we, what would be possible and where

are the limitations to apply science.

I would never have been able to finish my dissertation without the guidance of my

committee members, help from friends, and support from my family and wife.

7

SUMMARY

RESUMO.......................................................................................................................... 9

ABSTRACT .................................................................................................................... 11

1 INTRODUCTION ......................................................................................................... 13

References ..................................................................................................................... 14

2 LITERATURE REVIEW ........................................................................................ 17

2.1 Grazing managements ......................................................................................... 17

2.2 Effects of sward structure on forage intake .......................................................... 19

2.3 Effects of grazing management on animal performance ...................................... 22

2.4 Effects of supplementation on forage intake and ruminal fermentation ................ 24

References ..................................................................................................................... 27

3 FEEDING LOW LEVEL OF ENERGY SUPPLEMET FOR BEEF CATTLE AND

INTERACTIONS WITH GRAZING MANAGEMENT ....................................................... 41

Abstract .......................................................................................................................... 41

3.1 Introduction ........................................................................................................... 42

3.2 Materials and methods ......................................................................................... 43

3.2.1 Experimental area and animals ......................................................................... 43

3.2.2 Experimental treatments and management practices ....................................... 43

3.2.3 Feed Samples Collection, Chemical Analysis and Calculations ........................ 44

3.2.4 Rumen Fluid and Blood Samples ...................................................................... 46

3.2.5 Forage Intake and digestibility ........................................................................... 47

3.2.6 Animal Behavior ................................................................................................ 47

3.2.7 Microbial Protein Synthesis and Nitrogen Efficiency ......................................... 48

3.2.8 Rumen Fermentation Kinetics ........................................................................... 49

3.2.9 Statistical Analysis............................................................................................. 49

3.3 Results ................................................................................................................. 50

3.3.1 Pasture and Supplement Samples .................................................................... 50

3.3.2 Grazing Behavior and Feed Intake .................................................................... 51

3.3.3 Apparent digestibility ......................................................................................... 53

3.3.4 Rumen Fluid and Plasma Glucose .................................................................... 54

3.3.5 Microbial Protein Synthesis and Nitrogen Use .................................................. 55

3.3.6 Rumen Fermentation Kinetics ........................................................................... 56

3.4 Discussion ............................................................................................................ 57

3.4.1 Pasture and supplement samples ..................................................................... 57

8

3.4.2 Animal Behavior and Forage Intake .................................................................. 59

3.4.3 Rumen Fluid and Blood Samples ..................................................................... 63


3.4.5 Apparent digestibility and ruminal kinetics ........................................................ 66

3.5 Conclusions ......................................................................................................... 67

References .................................................................................................................... 67

4 FEEDING MEDIUM LEVEL OF ENERGY SUPPLEMET FOR BEEF CATTLE

AND INTERACTIONS WITH GRAZING MANAGEMENT .............................................. 85

Abstract .......................................................................................................................... 85

4.1 Introduction .......................................................................................................... 86

4.2 Materials and methods ......................................................................................... 87

4.2.1 Experimental area and animals ........................................................................ 87

4.2.2 Experimental treatments and management practices ....................................... 87

4.2.3 Feed Samples Collection, Chemical Analysis and Calculations ....................... 88

4.2.4 Rumen Fluid and Blood Samples ..................................................................... 89

4.2.5 Forage Intake and digestibility .......................................................................... 89

4.2.6 Animal Behavior ................................................................................................ 90


4.2.8 Rumen Fermentation Kinetics........................................................................... 91

4.2.9 Statistical Analysis ............................................................................................ 91

4.3 Results ................................................................................................................. 92

4.3.1 Pasture and Supplement Samples ................................................................... 92

4.3.2 Forage Intake and Animal Behavior .................................................................. 92

4.3.3 Apparent digestibility ......................................................................................... 96

4.3.4 Rumen Fluid and Plasma Glucose ................................................................... 97


4.4 Discussion ............................................................................................................ 99

4.4.1 Animal Behavior and Forage Intake .................................................................. 99

4.4.2 Rumen Fluid and Blood Samples ................................................................... 102

4.4.3 Microbial Protein Synthesis and Nitrogen Efficiency ....................................... 104

4.4.4 Apparent digestibility ....................................................................................... 104

4.5 Conclusions ....................................................................................................... 105

References .................................................................................................................. 105

9

RESUMO

Suplementação energética para bovinos mantidos em pastagem tropical (Brachiaria brizantha, cv Marandu) manejados em sistema de pastejo rotativo

com diferentes alturas de entrada

Dois experimentos foram conduzidos simultaneamente, para avaliar o uso da suplementação energética para bovinos manejados em diferentes alturas de entrada na pastagem. Foram usados 8 novilhos Nelore canulados no rumen por experimento (Exp. 1: 300 kg de PC ± 5,97, Exp. 2: 343 kg PC ± 7,40) distribuídos em 2 quadrados latinos 4x4. Os tratamentos para o Exp. 1 foram 0 (suplementação mineral) e 0,3 (0,3% do PC em milho moído) combinados com 2 alturas de entrada (25 e 35 cm). A altura de saída foi 15 cm. No Exp. 2 o nível de suplementação foi 0,6% do PC em milho moído. Os animais foram manejados em 2 ha de Capim Marandu, os quais foram adubados com 120 kg de N/há, apresentando valores médios de 13,8 e 11,0% de PB e 58,8 e 63,4% de FDN para pastos de 25 and 35 cm, respectivamente. A DMS e DPB da forragem e da dieta foram maiores (P<0,05) para o manejo da pastagem de 25 cm do que 35. Em ambos os experimentos, o CMS de forragem, energia e total foi maior (P<0,05) para o tratamento de 25 cm, que ao mesmo tempo promoveu menor tempo de pastejo (P<0,05), maior tempo em ócio (P<0,05) e taxa de bocado (P<0,05), menor número de passos por dia e passos entre estações de pastejo (P<0,05), quando comparados com animais mantidos no tratamento de 35 cm. O pH ruminal foi menor (P<0,05 no Exp. 1; P<0,10 no Exp. 2), a N-NH3 ruminal e retenção do N foram maiores (P<0,05) para animais manejados na altura de entrada de 25 cm. Os AGVs e a síntese microbiana não foram afetados (P>0,05) pelo manejo da pastagem. A suplementação em 0,3% (Exp. 1) aumentou (P<0,05) a DMS da dieta, enquanto a suplementação de 0,6% (Exp. 2) reduziu a DPB da forragem (P<0,05), aumentou a digestibilidade da FDN da forragem (P<0,05) e a DMS (P<0,05) e da FDN da dieta (P<0,01). A suplementação em 0,3% (Exp. 1) ou em 0,6% (Exp. 2) reduziu o CMS de foragem (P<0,05) e as taxas de substituição foram 1.63 and 0.72, respectivamente. O CMS total e de energia não foram aumentados (P>0,05) pela suplementação em 0,3%, enquanto o nível de 0,6% foi efetivo em aumentar o CMS total e de energia de bovinos mantidos em pastagem tropical, independente do manejo da pastagem. A suplementação reduziu o tempo de pastejo (P<0,05). Animais suplementados com 0,3% não alteraram (P>0,05) o pH ruminal, a retenção de N e síntese microbia, mas aumentaram (P<0,05) propionato no rumen e diminuíram (P<0,05) N-NH3 ruminal, acetato e relação acetato:propionato. A suplementação com 0,6% diminuiu (P<0,05) o pH ruminal, N-NH3 ruminal, acetato e relação acetato:propionato no rumen, aumentaram (P<0,05) o propionato no rumen, a retenção de N e a síntese microbiana. A glicose plasmática não foi alterada (P>0,05). A altura de pré-pastejo de 25 cm e a suplementação energética de 0,6% do PC foram estratégias eficientes para aumentar o consumo de energia de bovinos mantidos em pastagens de Capim Marandu. Palavras-chave: Energia; Forragem tropical; Gado de corte; Manejo da pastagem;

Suplementação

11

ABSTRACT

Energy supplementation for beef steers grazing tropical grass (Brachiaria brizantha, cv Marandu) managed under rotational system with different initial

sward heights

Two trials were conducted simultaneously to evaluate the effects of energy supplementation for cattle grazing tropical pastures managed with different initial sward heights on DMI and ruminal fermentation of cattle grazing intensively managed tropical grass during the rainy season. Eight 24-month-old rumen-cannulated Nellore steers were used per trial (Trial 1: 300 kg BW ± 5.97, Trial 2: 343 kg BW ± 7.40) allocated in two 4x4 Latin squares. Treatments corresponded to 0 (mineral supplementation) and a 0.3 (0.3% of BW of ground corn as fed basis) combined with 2 pre-grazing sward heights (25 and 35 cm). The stubble height was 15 cm. In the second trial the level of supplementation was 0.6% BW of ground corn as fed basis. Steers were managed in 2 ha of Palisadegrass pasture (Brachiaria brizantha marandu). Pastures were fertilized with 120 kg nitrogen/ha and averaged 13.8 and 11.0% CP and 58.8 and 63.4% NDF, for 25 and 35 cm, respectively The forage and the diet DM and CP digestibility were greater (P<0.05) for 25 then for 35 cm grazing management. For both trials 1 and 2, cattle grazing the pastures with 25 cm initial sward height consumed more forage DMI, more total DMI and more energy (P<0.05) and at the same time steers spent less time grazing (P<0.05) and more time resting (P<0.05), presented greater bite rates (P<0.05), less steps per day and less steps between feeding stations (P<0.05), when compared with cattle grazing the 35 cm pastures. Rumen pH values were less (P<0.05 in trial 1; P<0.1 in trial 2) and concentrations of rumen N-NH3 and retention of N were greater (P<0.05) for cattle grazing the 25 cm pastures while rumen VFA and microbial synthesis were not affected (P>0.05) by pasture management. Supplementing energy at 0.3% (trial 1) increased (P<0.05) diet DM digestibility while feeding energy at 0.6% (trial 2) decreased forage CP digestibility, increased (P<0.05) forage NDF digestibility and increased diet DM (P<0.05) and diet NDF (P<0.1) digestibility. Supplementing energy at 0.3% (trial 1) or at 0.6% (trial 2) decreased forage DMI (P<0.05) and substitution rates were 1.63 and 0.72, respectively. The total DMI and energy intake were not increased (P>0.05) by supplementing energy at 0.3% while increasing energy supplementation to 0.6% was effective to increase total DMI and energy intake of cattle grazing tropical forage, independent of initial sward height. Energy supplementation decreased (P<0.05) grazing time, but it did not affect (P>0.05) any other grazing behavior parameter. Supplementing grazing cattle with 0.3% had no effect (P>0.05) on rumen pH, N retention and microbial synthesis, increased (P<0.05) rumen propionate and decreased (P<0.05) rumen N-NH3, rumen acetate and acetate:propionate ratio. Supplementing grazing cattle with 0.6% decreased (P<0.05) rumen pH, rumen N-NH3, rumen acetate and acetate:propionate ratio, while it increased (P<0.05) rumen propionate, N retention and microbial synthesis. Plasma glucose was not affected by treatments (P>0.05). The pre-grazing sward height of 25 cm and feeding energy supplement at 0.6% of BW were efficient strategies to increase energy intake of cattle grazing Palisadegrass. Keywords: Beef cattle; Energy; Grazing management; Supplementation; Tropical

grass

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1 INTRODUCTION

About 92% of Brazilian beef production comes from grazing systems (MILLEN

et al., 2009) and the great majority of these grassland areas are covered with tropical

grasses. Even in intensive tropical grazing systems based on well managed

pastures, cattle ADG and milk production are far lower than animals genetic potential

(SANTOS et al., 2014). Most of this limitation comes from limited energy intake, due

to limitations in forage harvest capacity of the animals (SANTOS et al., 2009; DANÉS

et al., 2013) and rumen fill caused by fiber content and low particle fragility (ALLEN,

2000) of grasses.

There is no doubt on the importance of the physical mechanisms regulating

forage intake of cattle. However, for grazing cattle, before the forage can reach the

gastrointestinal tract, it has to be harvested by the animal. Hodgson et al. (1977)

reported that the structure of the sward may have a greater impact on forage intake

of grazing cattle than the physiological mechanisms discussed by Conrad et al (1964)

and Allen (2000). The structure of the sward has a great impact on the efficiency of

forage harvesting by grazing cattle, either for temperate grasses (HODGSON et al.,

1994) as for tropical grasses (DA SILVA; PEDREIRA, 1996; CARVALHO et al., 2001;

DA SILVA; CARVALHO, 2005; DA SILVA et al., 2009; PAULA et al., 2012).

The nitrogen fertilization increases the CP content in tropical grasses

(JONHSON et al., 2001; DANÉS et al., 2013) which may be adequate or even

excessive for growing cattle (NATIONAL RESEARCH COUNCIL - NRC, 1996).

However, their sward structure often limits forage harvesting (BENVENUTTI et al.,

2006) and its association with high NDF content (≥50%) (DANÉS et al., 2013) and

low particle fragility (ALLEN, 2000) causing rumen fill, limits forage intake and results

in energy being the most limiting nutrient. Thus, supplementing grain or by-products

high in energy is a strategy to overcome this limitation. However, energy

supplementation may decrease forage intake due to substitution effect (ELIZALDE et

al., 1998). This may not be related to rumen pH and its effects on fiber degradation

(CATON; DHUYVETTER, 1997). When substitution rate is high, total DMI and energy

intake may not be increased, resulting in no improvements in animal performance.

Dórea (2011) reported that supplementing ground corn at 0.3% BW for Nellore steers

on Palisadegrass cv. Marandú pasture managed under rotational grazing with 25 cm

canopy height (corresponding to 95% light interception) (TRINDADE, 2007) as the

14

start grazing point, did not increase cattle energy intake primarily due to a high

substitution rate. When supplement was fed at 0.6% BW, substitution rate decreased

and energy intake was increased. Correia (2006) reported a linear increase in cattle

ADG to energy supplementation (0.3 vs 0.6 vs 0.9% BW) on Palisadegrass cv.

Marandu pasture managed under rotational grazing with high stocking rates during

the rainy season. However, the response to the 0.3% BW treatment was quite small.

Feeding 0.6% BW resulted in extra ADG 3 times greater than 0.3%BW compared to

the no supplemented cattle.

Therefore, practices that improve energy intake by grazing cattle as

refinement in pasture management practices and energy supplementation may be

powerful tools to improve animal performance and productivity in tropical grazing

systems. However, the response to association of these two practices has not been

reported in the literature for cattle grazing tropical grasses.

In this context, the hypotheses of this study were: 1) Energy supplement fed at

0.3% BW may cause high substitution rate and consequent no increase in energy

intake for cattle on tropical pastures managed under rotational grazing; 2) Feeding an

energy supplement at levels greater than 0.3% BW may be required to increase

energy intake of cattle on tropical pastures managed under rotational grazing; 3)

Interactions may occur between energy supplementation and pasture management

practices; 4) the adoption of an ideal start grazing point based on canopy height

(correlated to the 95% light interception) may increase forage harvesting efficiency,

nutrient intake and metabolism of cattle grazing Palisadegrass cv. Marandu.

The propose of these studies were to evaluate the effects of energy

supplementation and its interaction with grazing management on cattle: 1) ingestive

behavior; 2) nutrient intake; 3) rumen parameters; 4) blood glucose; 5) apparent

digestibility; 6) rumen fermentation kinetics; and 7) nitrogen use.

References

ALLEN, M.S. Effects of diet on short-term regulation of feed intake by lactating dairy cattle. Journal of Dairy Science, Lancaster, v. 83, n. 7, p.1598-1624, 2000.

BENVENUTTI, M.A.; GORDON, I.J.; POPPI, D.P. The effect of the density and physical properties of grass stems on the for-aging behaviour and instantaneous intake rate by cattle grazing an artificial reproductive tropical sward. Grass and Forage Science, Oxford, v. 61, p. 272–281, 2006.

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CARVALHO, P.C.F.; RIBEIRO FILHO, H.M.N.; POLI, C.H.E.C.; MORAES, A.; DELAGARDE, R. Importância da estrutura da pastagem na ingestão e seleção de dietas pelo animal em pastejo. In: REUNIÃO ANUAL DA SOCIEDADE BRASILEIRA DE ZOOTECNIA, 38., 2001. A produção animal na visão dos brasileiros: anais... Piracicaba: SBZ, 2001. v. 1, p. 853-871.

CATON, J.S.; DHUYVETTER, D.V. Influence of energy supplementation on grazing ruminants: requirements and responses. Journal of Animal Science, Savoy, v. 75, n. 2, p. 533, 1997.

CONRAD, H.R.; PRATT, A.D.; HIBBS, J.W. Regulation of feed intake in dairy cows. I. Change in importance of physical and physiological factors with increasing digestibility. Journal of Dairy Science, Lancaster, v. 47, n. 1, p. 54-62, 1964.

DA SILVA, S.C.; CARVALHO, P.C.F. Foraging behavior and herbage intake in the favorable tropics/sub-tropics. In: McGILLOWAY, D.A. (Org.). Grassland: a global resource. Wageningen: Wageningen Academic Publ., 2005, p. 81-95.

DA SILVA, S.C.; PEDREIRA, C.G.S. Fatores predisponentes e condicionantes da produção animal a pasto. In: SIMPÓSIO SOBRE MANEJO DA PASTAGEM, 13., 1996, Piracicaba. Produção de bovinos a pasto: anais... Piracicaba: FEALQ, 1996. p. 319-352.

DA SILVA, S.C.; BUENO, A.A.O.; CARNEVALLI, R.A.; UEBELE, M.C.; BUENO, F.O.; HODGSON, J.; MATTHEW, C.; ARNOLD, G.C.; MORAIS, J.P. Sward structural characteristics and herbage accumulation of Panicum maximum cv Mombaça subjected to rotational stocking managements. Scientia Agrícola, Piracicaba, v. 66, p. 8-19, 2009.

DANÉS, M.A.C. Teor de proteína no concentrado de vacas em lactação mantidas em pastagens de capim elefante. 2007. 99 p. Dissertação (Mestrado em Ciência Animal e Pastagens) – Escola Superior de Agricultura “Luiz de Queiroz”, Universidade de São Paulo, Piracicaba, 2010.

DANÉS, M.A.C.; CHAGAS, L.J.; PEDROSO, A.M.; SANTOS, F.A.P. Effect of protein supplementation on milk production and metabolism of dairy cows grazing tropical grass. Journal of Dairy Science, Lancaster, v. 96, p. 407-419, 2013. DOREA, J.R.R. Níveis de suplemento energético para bovinos em pastagens tropicais e seus efeitos no consumo de forragem e fermentação ruminal. 2011. 108 p. Dissertação (Mestrado em Ciência Animal e Pastagens) - Escola Superior de Agricultura “Luiz de Queiroz”, Universidade de São Paulo, PIiracicaba, 2011.

ELIZALDE, J.C.; CREMIN, J.D. Jr.; FAULKNER, D.B.; MERCHEN, N.R. Performance and digestion by steers grazing tall fescue and supplemented with energy and protein. Journal of Animal Science, Savoy, v. 76, p. 1691, 1998.

HODGSON, J., CLARK, D.A.; MITCHELL, R.J. Foraging behavior in grazing animals and its impact on plant communities. In: FAHEY, G.C. (Ed.). Forage quality, evaluation and utilization. Lincoln: American Society of Agronomy, 1994. p. 796-827.

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HODGSON, J.; RODRIGUEZ CAPRILES, J.M.; FENLON, J.S. The influence of sward characteristics on the herbage intake of grazing calves. Journal of Agricultural Science, Cambridge, v. 89, n. 2, p. 743-750, 1977.

JOHNSON, C.R.; REILING, B.A.; MISLEVY, P.; HALL, M.B. Effects of nitrogen fertilization and harvest date on yield, digestibility, fiber, and protein fractions of tropical grasses. Journal of Animal Science, Albany, v. 79, p. 2439-2448, 2001.

MILLEN, D.D.; PACHECO, R.D.L; ARRIGONI, M.D.B.; GAYLEAN, M.L.; VASCONCELOS, J.T. A snapshot of management practices and nutritional recommendations used by feedlot nutritionists in Brazil. Journal of Animal Science, Savoy, v. 87, p. 3427-3439, 2009.

PAULA, C.C.L.; EUCLIDES, V.P.B.; MONTAGNER, D.B.; LEMPP, B.; DIFANTE, G.S.; CARLOTO, M.N. Estrutura do dossel, consumo e desempenho animal em pastos de capim-marandu sob lotação contínua. Arquivo Brasileiro de Zootecnia e Medicina Veterinária, Belo Horizonte, v. 64, n. 1, p. 169-176, 2012.

SANTOS, F.A.P.; DOREA, J.R.R.; AGOSTINHO NETO, L.R.D. Uso estratégico da suplementação concentrada em sistemas de produção animal em pastagens. In: SIMPÓSIO SOBRE MANEJO DA PASTAGEM, 25., 2009, Piracicaba. Anais... Piracicaba: FEALQ, 2009. p. 273-296.

SANTOS, F.A.P.; DÓREA, J.R.R.; SOUZA, J.; BATISTEL.; COSTA, D.F.A. Forage management and methods to improve nutrient intake in grazing cattle. In: FLORIDA RUMINANT NUTRITION SYMPOSIUM, 25., 2014, Gainesville, Proceedings… [S.l.: s. n.], 2014. p. 144-165.

TRINDADE, J.K. Modificações na estrutura do pasto e no comportamento digestivo de bovinos durante o rebaixamento do capim-marandu submetido a estratégias de pastejo rotacionado. 2007. 162 p. Dissertação (Mestrado em Ciência Animal e Pastagem) - Escola Superior de Agricultura “Luiz de Queiroz”, Universidade de São Paulo, Piracicaba, 2007.

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2 LITERATURE REVIEW

2.1 Grazing managements

The structure of the sward has a great impact on the efficiency of forage

harvesting by grazing cattle, either for temperate grasses (HODGSON et al., 1994)

as for tropical grasses (DA SILVA; PEDREIRA, 1996; CARVALHO et al., 2001; DA

SILVA, 2009).

Just after the grazing process, the grass initiates its regrowth with intense leaf

appearance and leaf growth and minimal stem elongation. The sward will reach a

determined height where light will start to become limiting for the lower portion of the

sward. Light restriction will stimulate leaves senescence and stem elongation. The

result is an increased proportion of senescent material and stems and a decreased

proportion of green leaves in the sward, with dramatic alteration on the grazing

efficiency and forage intake (DÓREA et al., 2013a, 2013b).

Rotational grazing based on fixed or pre-established number of days for grazing

intervals has been criticized by Da Silva and Corsi (2003). Grass dry matter

production is dictated by its genetic potential and by environmental conditions such

as temperature, sun light, soil fertility and water availability. As these conditions vary,

grazing interval must vary too. The ideal start grazing point should be based on plant

physiology aspects instead of on fixed grazing intervals.

The concept of an ideal start grazing point originated from studies with

temperate grasses (HODGSON, 1990) using 95% light interception (LI) criterion has

been successfully applied to tropical grasses (DA SILVA; NASCIMENTO JÚNIOR,

2007). The latter authors reported a number of studies conducted in Brazil with

various tropical grasses and a range of sward height values correlated to 95% LI.

The studies reported by Da Silva and Nascimento Júnior (2007) indicated that the

sward heights correlating to 95% LI vary with time of the year, but a much higher

degree of variation is between plant spp. Some of the work reviewed correlating 95%

LI to sward height are presented in Table 1.

18

Table 1 - Suggested sward and stubble heights for pastures managed based on 95%

light interception (LI)

Plant spp. Sward height (cm)

Reference Entrance Exit

Mombaça 90 30 to 50 Carnevalliet al., (2006)

Tanzânia 70 30 to 50 Difante, (2005); Barbosa et al., (2007) Xaraés 30 15 to 20 Pedreira, (2006) Cameroon 100 40 to 50 Voltolini, (2006); Carareto, (2007)

Marandu 25 10 to 15 Zeferino, (2006); Sarmento, (2007); Costa (2007); Souza Júnior, (2007); Trindade, (2007)

Tifton-85 25 10 to 15 Da Silva and Corsi (2003) Coastcross 30 10 to 15 Da Silva and Corsi (2003)

Mombaça = Panicum maximum cv. Mombaça; Tanzânia = Panicum maximum cv. Tanzânia; Xaraés = Brachiaria brizantha cv. Xaraés; Cameroon = Pennisetum purpureum cv. Cameroon; Marandu = Brachiaria brizantha cv. Marandu; Tifton-85 = Cynodon dactylon cv. Tifton-85; Coastcross = Cynodon dactylon cv. Coastcross

Morphological composition of plants managed based on 95% light interception

(LI) criterion as the ideal start grazing point present significantly higher proportion of

leaves and lower proportion of stem and senescent material in comparison to plants

managed with fixed grazing intervals (Table 2).

Table 2 - Morphological composition of plants managed either based on 95% light

interception (LI) criterion or on fixed grazing intervals

Reference Forage Grazing frequency Leaves % DM

Stem % DM

Senescent material % DM

Voltolini (2006) Cameroon 27 fixed days 48.0 46.0 6.0 Voltolini (2006) Cameroon 95% LI 53.0 42.0 5.0 Correia (2006) Marandu 21 fixed days 34 32.7 33.3 Costa (2007) Marandu 95% LI 56 32 12 Cameroon = Pennisetum purpureum cv. Cameroon; Marandu = Brachiaria brizantha cv. Marandu

Carvalho et al. (2001), pointed out that the structure of the sward would have a

greater importance in forage intake regulation for grazing cattle than the physiological

and chemical mechanisms discussed by Conrad et al. (1964) and Allen (2000) due to

its impact on the harvesting process of forage. The impact of the sward structure on

forage intake of grazing cattle has been reported by several other authors (DA

19

SILVA; CARVALHO, 2005; CASAGRANDE, 2010; VIEIRA, 2011; PAULA et al.,

2012).

2.2 Effects of sward structure on forage intake

Sward structure can be described in terms of forage mass, pasture height,

ground cover, and forage density for different strata (CANGIANO et al., 2002).

Variations in any of these variables may affect bite density, bite area and bite mass

(HODGSON, 1990).

Considering that daily forage intake can be defined as the product of bite

mass, bite rate, and grazing time (Allden and Whittaker 1970; Hodgson 1981), then

the sward structure can alter significantly the forage intake, and in the most cases for

animals in grazing systems can affect the consume more than physical and

physiological mechanisms (HODGOSON, 1977; CANGIANO et al., 2002).

Forage intake is a function of body weight (BW0.75) in growing animals, but

under grazing conditions intake may be limited by sward characteristics (CANGIANO

et al., 2002). Illius and Gordon (1987) described a model in which the allometric

relationship between BW and bite area changes with sward height. On short swards,

where only a narrow band of tillers can be prehended, bite area is determined by

incisor arcade breadth (BW0.36). In tall swards, where the maximum bite area can be

achieved, bite area is related to the square of the incisor arcade breadth, so it scales

to BW0.72.

To explain and quantify forage intake, a many mechanisms regarding the

dynamics of the plant–animal interface must be detailed and understood. Because

increases in bite rate or grazing time generally do not compensate for reductions in

bite mass, it is the most important variable in daily intake (CHACON; STOBBS,

1976). Bite mass is the product of bite area, bite depth, and pasture bulk density in

the grazing horizon (BURLISON; ILLUS, 1991).

There is a historical idea that lower animal production in tropical pastures

would be related with low forage quality, as reported by Sollenberger and Burns

(2001), where the authors pointed out that tropical pastures produce low-quality

forage with high bulk density of pseudostems, and just would support low levels of

animal performance.

20

However, several authors (DA SILVA; PEDREIRA, 1996; CARVALHO et al.,

2001; DA SILVA; CARVALHO, 2005; DA SILVA et al., 2009; PAULA et al., 2012)

have worked for many years to support an opposite idea, where these authors

discussed and concluded that pasture structure was more important in constraining

forage intake than previously supposed. In fact, basing pasture management on

degree of sward light interception and avoiding stem development has supported

new management strategies (TRINDADE, 2007; GIMENES et al., 2011; DANÉS et

al., 2013), resulting in unexpected high levels of animal production (CARARETO,

2007; VOLTOLINI et al., 2010; GIMENES et al., 2011).

In the important and recent meta-analysis published by Carvalho (2013) was

demonstrated how tropical pasture structure influences forage intake. Carvalho

(2013) suggests that grazing animals take more time to gather a given bite mass in

tropical than in temperate pastures. In the graphic 1 (CARVALHO, 2013), is possible

to observe the intercept of the model refers to the time to prehend the bite,

independently of bite mass, what indicate that mistakes on grazing managements in

tropical grass may affect drastically DMI when compared with temperate grasses.

21

Graphic 1 - Temperate pastures (○, solid line): 1 – Lolium multiflorum (Amaral et al. 2013); 2 – Avena strigosa sward under continuous, and 3 – rotational stocking; 4 – Lolium multiflorum, Avena strigosa and avena + ryegrass mixture (G.C. Guzatti comm.). Tropical pastures (●, dotted line): 5 – Cynodon sp. under rotational, and 6 – continuous stocking; 7 – Sorghum bicolor under rotational, and 8 – continuous stocking; 9 – Brachiaria brizantha under rotational stocking ; 10 – natural grassland under continuous stocking ; 11 – Pennisetun glaucum under rotational stocking. Regression equations have been generated for each species in each experiment, and then compared by parallelism test and equality of intercepts (P<0.05). There are no differences between stocking methods in each group of pastures. Temperate pastures model: y = 0.457x + 0.800; R2 = 0.724; P<0.0001; s.e. = 0.142; n = 98. Tropical pastures model: y = 0.395x + 1.166; R2 = 0.489; P<0.0001; s.e. = 0.239; n = 185

Source: Carvalho (2013)

There are many references in the literature showing high contents of NDF in

tropical forages (MORAIS et al., 2009; SALES et al., 2009; PORTO et al., 2009) ,

what associated with inadequate management can intensify troubles on forage

intake.

According to Hodgson et al. (1994), the forage quality is usually higher in leaves than

in stems and it is generally accepted that animals select leaves and avoid stems.

However, recent studies on artificial swards have shown that this selective behavior

varies according to the tensile resistance of the stems (BENVENUTTI et al., 2006,

2008). The animals did not select against stem with low tensile resistance but

avoided stems of high tensile resistance. As a result, diet quality increased in relation

to the quality of the forage on offer but intake rate decreased with the increase of

tensile resistance of the stem component of the sward.

In swards with a stem-dominated lower stratum and a leaf-dominated upper

stratum, the lower stratum can form a vertical barrier reducing bite depth (FLORES et

al., 1993; BENVENUTTI et al., 2006, 2009).

22

It is easy to find in Brazilian conditions, environments where the criteria to start

graze is lost, and the plant maturity increases. In that point, the NDF content

increases and the nutritive value associated with more stem tensile resistance may

contribute to decrease DMI.

The animal is able to choose the ideal point to graze, but some changes occur

among the grazing process, regarding maximizes the forage intake and to do that,

the animal is obligate to change for another point to graze. This point is called

feeding station (FS), and is defined as the area of pasture a grazing animal can

reach at each eating step. The eating step is taken while eating, considering that

grazing activities involves eating and searching (SEARLE et al., 2005).

The bite profitability is defined as digestible energy harvested with each bite

per handling time per bite (FORTIN, 2001). The decisions regarding the profitability of

each bite influence departure decisions or residence time per FS (SEARLE et al.,

2005). According to Wade and Gregorini (2010), this decline and the perception of

FS more profitable in the surrounds motivate the animal to move to a new FS.

Therefore, a bite of forage should be more profitable to a hungry animal than

to one that is not hungry (NEWMAN et al., 1994).

2.3 Effects of grazing management on animal performance

There are a limited number of performance trials where the 95% light

interception criterion was compared to fixed grazing interval with tropical grasses.

Gimenes et al. (2011) compared two sward heights as the start grazing point,

25 cm (95% light interception) and 35 cm (close to 100% light interception) for

Nellore yearling bulls grazing Brachiaria brizantha cultivar Marandu (Palisade grass)

(Table 3). In both treatments pastures were grazed down to 15 cm height.

http://www.sciencedirect.com/science/article/pii/S1871141310005809#bb0095




23

Table 3 - Effect of start grazing point (25 vs 35 cm sward height) on performance of

Nellore yearling bulls grazing Palisade grass (Brachiaria Brizantha cv.

Marandu)

Sward height ADG

(kg/animal) Stocking rate

(450 kg AU/ha) Kg LWG/ha

25 cm 0,957 2,88 621

35 cm 0,769 2,43 401

Average values for summer and fall season. Gimenes et al. (2011)

Animals grazing the pastures managed based on the 95% light interception

criterion (25 cm as the grazing starting point) presented greater ADG and the

pastures supported greater stocking rate. The fine tuning on pasture management

resulted in 55% increase in BWG/ha.

Voltolini et al. (2010) reported higher milk production, higher stocking rates and

consequently higher productivity (production per area) when cows grazed pastures of

Elephant grass cv. Cameroon managed using 95% LI (103 cm sward height) as the

grazing starting point in comparison with cows grazing paddocks managed with 27

days fixed grazing intervals (Table 4). Post grazing sward heights were 62 and 71 cm

for the 95% LI and for the 27 d fixed GI respectively.

Table 4 - Effect of start grazing point (103 cm of sward height vs 27 days fixed

grazing interval) on performance of dairy cows grazing Elephant grass cv.

Camerron

27 days of

grazing interval

95% LI 1,03 m of sward

height P value

3.5% FCM, kg. cow d-1 14.88 17.65 0.1000

Cows.ha-1 5.1 7.2 0.0020

Milk, kg.ha.d-1 75 114 0.0004

Voltolini et al. (2010)

Carvalho et al. (2001), Da Silva and Carvalho (2005), Casagrande (2010),

Vieira (2011) and de Paula et al. (2012) reported greater DMI of cattle grazing

24

pastures with better sward structure. Carvalho (1997) and Pedreira et al. (2005)

reported that cattle grazing pastures with same initial forage mass but with different

sward structures may have different forage intake and performance.

2.4 Effects of supplementation on forage intake and ruminal fermentation

Energy supplementation is often practiced during summer or wet season,

where the forage has higher CP content compared with winter or dry season. In well

managed tropical grass, with high doses of nitrogen fertilization and harvest at the

ideal point (LI concepts) the CP content may be increased and in most cases the

level of CP is enough to supply CP requirements for a growing beef cattle, being the

energy the most limiting nutrient, since it is not possible to reduce substantially the

NDF content, that vary from 54.2 to 66.3% (Table 5).

Table 5 - Chemical composition (% DM) of hand plucked or grazing horizon samples

of well managed tropical pastures collected during spring, summer and fall

Forage CP1, % NDF2, % Reference

Brachiaria brizantha cv. Marandu 12. 6 57. 4 Correia (2006)

Brachiaria brizantha cv. Marandu 13. 6 56. 2 Correia (2006)

Brachiaria brizantha cv. Marandu 15. 3 65. 0 Costa (2007)

Brachiaria brizantha cv. Marandu 15. 4 63. 9 Pacheco, jr. (2009)

Brachiaria brizantha cv. Marandu 11. 9 66. 3 Agostinho neto (2010)

Brachiaria brizantha cv. Marandu 13. 1 62. 6 Dórea (2011)

Pennisetum purpureum cv. Cameroon 13. 7 62. 9 Martinez (2004)

Pennisetum purpureum cv. Cameroon 14. 6 65. 1 Voltolini (2006)

Pennisetum purpureum cv. Cameroon 20. 6 63. 2 Carareto (2007)

Pennisetum purpureum cv. Cameroon 17. 6 64. 4 Romero (2008)

Pennisetum purpureum cv. Cameroon 18. 5 61. 4 Martinez (2008)

Pennisetum purpureum cv. Cameroon 18. 5 58. 7 Danés (2010)

Pennisetum purpureum cv. Cameroon 15. 5 60. 2 Chagas (2011)

Pennisetum purpureum cv. Cameroon 18. 6 54. 4 Macedo (2012)

Pennisetum purpureum cv. Cameroon 18.3 54.2 Souza (2014)

1 CP = Crude protein;

2NDF = Neutral Detergent Fiber

25

According to Danés et al. (2013) and NRC (1996) cattle grazing only tropical

grass normally are under imbalance between protein and fermentable carbohydrate.

In well managed tropical grass, as mentioned above (Table 6), this fact it is

intensified, because the CP content increases, but the NDF does not decrease in

order to provide improvements in microbial synthesis as resulted of more available

energy into the rumen (SANTOS et al., 2014).

Supply additional energy through concentrate supplement is a powerful tool to

improve energy intake, minimizing the mentioned unbalance between protein and

energy. However, providing additional energy in the form of supplement has often

produced reductions in intake of grazed forage. Chase and Hibberd (1987) fed

incremental levels of corn to cows consuming low-quality forage and reported linear

decreases in forage OM intake. These results support observations from earlier work

on energy supplementation (LUSBY; WAGNER, 1986). After that, Pordomingo et al.

(1991) reported that cattle supplemented with corn while grazing summer pasture

had reduced forage intakes. These reports above agree with other data from tropical

and temperate forages (MINSON, 1990).

Matejovsky and Sanson (1995) and Henning et al. (1990) reported that low

levels of energy supplementation for sheep increased intake. In general, as level of

energy supplement provided increases, intake usually decreases. According to Caton

and Dhuyvetter (1997) low levels of energy supplementation increase forage intake

seem to occur much more frequently in studies with sheep than in those with cattle.

Reductions in forage intake associated with corn supplementation have been

attributed to starch (CATON; DHUYVETTER, 1997). The increases on levels of corn

starch decreased forage intake in steers (SANSON et al., 1990). These reductions

have been attributed to either depressions in ruminal pH or a carbohydrate effect

(MOULD et al., 1983). Declining ruminal pH associated with increasing dietary starch

should affect the ruminal bacteria toward greater amylolytic and lower cellulolytic

population. Resulting bacterial shifts are thought to reduce fiber digestion and

negatively affect intake of grazed forage.

These hypotheses initially mentioned by many nutritionists that energy

supplementation reduces rumen pH, affecting fiber digestion and consequently

forage DMI is not consistent according to reviews published by Caton and Dhuyvetter

(1997) and Santos et al. (2009). Studies regarding the factors that regulates

26

voluntary intake have been focus by many researchers during the last years (ANIL;

FORBES, 1988; ALLEN et al., 2009; ZIEBA et al., 2005).

Energy supplementation is responsible for increasing volatile fatty acids, such

as propionate, what is pointed out as one of the responsible to modulate dry matter

intake in ruminants. The hypophagic effect of propionate in ruminants has been

widely documented (ALLEN et al., 2009). However, information regarding the exact

mechanisms of propionate in the liver and its action on the hungry control is scarce.

The voluntary intake can be regulated by hormones that affect the satiety

status, such as insulin, leptin and ghrelin, secreted by pancreas, adipose tissues and

stomach (abomasum in ruminants). These hormones are sensible to diet

manipulation, and its believe that they play an important role in front of to send

signals for central nervous system regarding the energy status and then regulates the

DMI (ZIEBA et al., 2005).

For example, Ghrelin has recently been suggested as a powerful, peripherally

active, orexigenic agent (WREN et al., 2000; KOBELT et al., 2006; ROCHE et al.,

2007). Plasma ghrelin concentration increases in response to fasting and decreases

during subsequent feeding in humans (CUMMINGS et al., 2001) and ruminants

(HAYASHIDA et al., 2001; WERTZ-LUTZ et al., 2006; ROCHE et al., 2007). The

orexigenic property of ghrelin may be involved in determining feeding mechanisms

and behavior (NAKAZATO et al., 2001; SUGINO et al., 2004) as well as energy

balance (HOSADA et al., 2002), the latter being related to glucose and insulin

metabolism (MENDEZ et al., 2006; TAKAHASHI et al., 2006). The positive

relationship between plasma ghrelin concentrations and DMI has been demonstrated

in confined sheep (MENDEZ et al., 2006).

Gregorini et al. (2009) conducted a study aiming to investigate changes in

foraging behavior, hunger-related hormones, and metabolites of dairy cows in

response to short-term variations in rumen fill. These authors found that when RF

was decreased, the glucose levels declined and ghrelin increased. Insulin acts as

both a short- and long-term regulator of feed intake (WOODS et al., 1979). The

results of Gregorini et al (2009) study add to this effect, directly or indirectly relating

the physical stimuli coming from the rumen to the role of this hormone in intake

regulation.

Gregorini et al. (2009) found correlation between insulin concentration and bite

depth and tended to be associated with bite mass. This fact demonstrates that many

27

alterations on DMI in grazing animals are not caused just by rumen fermentation, but

by hormonal regulation that can affect the grazing behavior and then de DMI. Thus,

for many years the rumen fermentation was the focus of possible explanations to

reductions on dry matter intake in supplemented animals, even in normal conditions

(rumen pH, ammonia, level of CP, etc.). The Dr. Gregorini's group has indicated that

cattle respond to an energy acquisition stimulus by changing ingestive behavior, what

many times is resulted of interactions between energy status and grazing

management, which has the hormones as an important regulators.

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3 FEEDING LOW LEVEL OF ENERGY SUPPLEMET FOR BEEF CATTLE AND

INTERACTIONS WITH GRAZING MANAGEMENT

Abstract

Eight 24-month-old rumen-cannulated Nellore steers (300 kg BW ± 5.97) were used in two 4x4 Latin squares to evaluate the interaction between energy supplementation and pre-grazing sward height on cattle grazing behavior, nutrient intake, digestion and metabolism. Treatments corresponded to 0 (mineral supplementation) or 0.3% (0.3% of BW of ground corn on a DM basis) combined with 2 pre-grazing sward heights (25 and 35 cm), with a common stubble height of 15 cm for all treatments. Steers were managed on 2 ha of Palisadegrass pasture (Brachiaria brizantha cv. Marandu) from February to April of 2011. Pastures were fertilized with 120 kg of nitrogen per ha and averaged 13.8 and 11.0% CP and 58.8 and 63.4% NDF, for the 25 and 35 cm grazing treatments, respectively. The parameters evaluated were forage intake, rumen pH and N-NH3, microbial synthesis, VFA, blood glucose, nitrogen balance and grazing behavior. Forage DMI and apparent DM, CP and NDF digestibility were obtained using Cr2O5 and indigestible NDF as digesta markers. Concentration of purine derivative in the urine was used to estimate microbial synthesis. Forage DMI was increased (1.86 vs 1.32 P<0.05) when pre-grazing sward height was 25 cm and decreased when 0.3% BW supplement was fed (1.79 vs 1.38, P<0.05). The total and digestible DMI were not affected by energy supplementation (P>0.05), however they were increased (P<0.05) when pre-grazing sward height was 25 cm. Steers grazing the 25 cm treatment spent significantly less time grazing and more time resting (P<0.05), took less steps between feeding stations and per day (P<0.05) and presented greater bite rates (P<0.05) when compared with 35 cm treatments. Energy supplementation decreased grazing time (P<0.05), but it did not affect any other grazing behavior parameter. The DM and CP digestibility of forage and of total diet were greater (P<0.05) for the 25 than the 35 cm grazing management. Energy supplementation increased (P<0.05) diet DM digestibility, without effects on CP and NDF digestibility (P>0.05). Rumen pH (6.39 vs 6.52) was less (P<0.05) and rumen N-NH3 (11.22 vs 9.77 mg/dL) was greater (P<0.05) for the 25 cm grazing management. Supplementation decreased (P<0.05) rumen N-NH3. Microbial synthesis was not affected (P>0.05) by treatments. The nitrogen retention was increased (P<0.05) for the 25 cm grazing management and not affected by supplementation (P>0.05). Concentration of total VFA was not affected by treatments (P>0.05). However, the energy supplementation decreased rumen acetate and increased propionate (mol/100 mol), resulting in decreased A:P ratio (P<0.05). Blood glucose was not affected by treatments (P>0.05). The pre-grazing sward height of 25 cm was effective to improve cattle harvesting efficiency and energy intake, but there was no benefit of feeding low level of energy supplement on energy intake of cattle on tropical pasture under rotational grazing. Keywords: Beef cattle; Energy; Grazing; Management; Supplementation; Tropical

grass

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3.1 Introduction

Forage intake is regulated by physical effect or rumen fill, or physiological

effect. For grazing cattle, where the most part of the diet is composed by fiber, the

physical regulation is the major mechanism controlling forage intake (CONRAD et al.,

1964).

However, for grazing cattle, before the forage can reach the gastrointestinal

tract, it has to be harvested by the animal (CANGIANO et al., 2002). The data

compilation done by Hodgson et al. (1977) suggested that the structure of the sward

may have a greater impact on forage intake of grazing cattle than the physiological

mechanisms discussed by Conrad et al. (1964) and Allen (2000).

The structure of the sward has a great impact on the efficiency of forage

harvesting by grazing cattle, either for temperate grasses (HODGSON et al., 1994)

as for tropical grasses (DA SILVA; PEDREIRA, 1996; DA SILVA; CARVALHO et al.,

2001; CARVALHO, 2005; DA SILVA et al., 2009; PAULA et al., 2012). An important

point reported by Pedreira et al. (2005) is that cattle grazing pastures with same initial

forage mass but with different sward structures may have different forage intake and

performance.

The concept of an ideal start grazing point originated from studies with

temperate grasses (HODGSON, 1990) using 95% light interception (LI) criterion has

been successfully applied to tropical grasses (DA SILVA; NASCIMENTO JÚNIOR,

2007). The latter authors reported a number of studies conducted in Brazil with

various tropical grasses and a range of sward height values correlated to 95% LI

(CARNEVALLI et al., 2006; BARBOSA et al., 2007; TRINDADE, 2007). According to

(DA SILVA; NASCIMENTO JÚNIOR, 2007) pastures managed based on the 95% LI

criterion present a sward that allows cattle to harvest greater amount of forage in a

shorter grazing period.

Even in intensive tropical grazing systems based on well managed pastures

as discussed above, animal performance is still far lower than animal’s genetic

potential. Most of this limitation comes from limited energy intake (DANÉS et al.,

2013), due to limitations in forage harvest capacity of the animals and rumen fill

caused by fiber content of the tropical grasses (WILSON, 1994). Feeding

concentrates high in energy is a powerful tool to increase energy intake, stocking rate

and cattle performance (VENDRAMINI et al., 2006, 2007; DA CRUZ et al., 2009;

43

FERNANDES et al., 2010). However interactions between energy supplementation

and grazing management have not been studied for tropical grasses. Therefore, the

objectives of this study were to evaluate the effects of feeding an energy supplement

at 0 or 0.3% BW and its interaction with grazing management on cattle: 1) ingestive

behavior; 2) nutrient intake; 3) rumen parameters; 4) blood glucose; 5) apparent

digestibility; 6) rumen fermentation kinetics; and 7) nitrogen use.

3.2 Materials and methods

This article is the first of a set of 2 experiments that evaluated interactions

between energy supplementation and grazing management. This article used a low

level of energy supplementation (0.3% BW), while in the companion article a higher

level of energy supplementation (0.6% BW) was tested. The experiments had

independent animals, but were carried out simultaneously in the same grazing area.

Thus, the forage chemical and morphological composition were the same for both

and will be presented in the current paper.

3.2.1 Experimental area and animals

The experiment was conducted at the Department of Animal Science of the

University of São Paulo (USP), located in Piracicaba-SP, Brazil, during the months of

February to April of 2011. The experimental area consisted of 2 ha of Palisadegrass

cv marandu managed as a rotational grazing system divided in 16 paddocks and 2

treatments (25 and 35 cm pre-grazing sward height). Eight 24-month-old Nellore

steers, 300 kg BW ± 5.97, cannulated in the rumen were used in this study.

3.2.2 Experimental treatments and management practices

Treatments consisted of 2 levels of energy supplementation (mineral

supplementation or 0.3% BW of ground corn as fed basis combined with 2 pre-

grazing sward heights (25 and 35 cm), with a common stubble height of 15 cm for all

treatments. Pasture management was performed as a rotational grazing system, with

use of nitrogen fertilizer (40 kg/ha per grazing cycle). The targeted pre-grazing sward

height of 25 and 35 cm was based on research with Palisadegrass cv. Marandu

correlating these measurements to 95 and 100% light interception (TRINDADE,

2007).

44

The experiment lasted 64 d and consisted of four experimental periods of 16 d

each. Animals were weighed (full BW) prior to the start of each experimental period

and the allocation of supplement was fixed for that experimental period. The first 8 d

were used to adapt the animals to the experimental treatments. The following 8 d

consisted of sample collections in the following order: on days 9 to 13 the fecal

samples collection was performed, on day 14 measurements of animal behavior were

taken, on day 15 collections of blood and urine samples were performed and on day

16 rumen fluid samples were collected. Animals had free access to water and

minerals supplement in every paddock. The supplemented and the no supplemented

experimental animals were brought daily to a management center close to the

experimental area where the supplement was offered in individual feed bunks from

1000h to 1040h. The no supplemented steers were also contained in the feed bunks.

This management center was also utilized for infusions of external marker and

samplings.

3.2.3 Feed Samples Collection, Chemical Analysis and Calculations

Forage mass and its morphological composition were determined in every

grazing cycle, before the steers entered every paddock and after they left. In each

evaluation, 3 randomized points were selected in the paddock, and in each point, the

material within a rectangle frame (0.5 m2) was cut to ground level. Total forage mass

was weighted, and a representative subsample of 0.5 kg was taken and separated

into leaf blades, stems (including leaves sheaths) and dead material (as indicated by

more than 50% of the tissue area being senescent) to determine the sward

morphological composition. After collection all samples were dried at 55º C. Pre and

post-grazing sward height, forage mass, and morphological composition are

presented in Table 6.

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Table 6 - Forage mass, sward height and morphological composition of pre and post-

grazing condition

Pre-grazing conditions

Pre-grazing sward height

SEM 25 cm 35 cm

Forage mass, kg DM/ha 8036.09 8899.75 530.36

Leaves, kg DM/há 3567.22 3306.59 194.67

Stems, kg DM/há 1405.51 1609.00 114.48

Dead material, kg DM/ha 3063.36 3984.16 457.21

Leaves, % 44.39 37.88 2.60

Stems, % 17.49 18.20 1.05

Dead material, % 38.12 43.91 2.96

Post-grazing conditions

Forage mass, kg DM/ha 4331.02 4704.38 529.99

Leaves, kg DM/há 990.23 787.37 148.55

Stems, kg DM/há 782.78 1046.43 151.42

Dead material, kg DM/ha 2558.01 2870.53 373.39

Leaves, % 22.89 16.85 2.30

Stems, % 18.09 22.39 2.37

Dead material, % 59.12 61.42 3.63

Sward height

Pre-grazing, cm 26.01 37.21 1.63

Post-grazing, cm 14.61 17.45 0.93

1Standard error of the means

The forage mass on pre-grazing conditions for the 25 cm treatment was

around 900 kg/ha greater than values reported in the literature for the same grass,

grazing management and season (SOUZA JÚNIOR, 2007; TRINDADE, 2007). The

mass on the pre and post-grazing conditions can vary depending on the adopted

management before the start of the trial, such as level of nitrogen fertilization,

season, stocking rate, stubble height, and other factors.

For pastures managed at 25 cm of pre-grazing sward height, the leaves

disappearance was 2,576.99 kg DM/ha, while at 35 cm it was 2,519.22 kg/ha.

Despite the similar amounts of leaves disappearing from both pasture treatments,

46

forage DMI may be less and forage loss may be greater for the 35 cm, considering

that more stems may contribute as horizontal barrier, decreasing bite area, bite rate

and consequently forage intake (DRESCHER et al., 2006; BAGGIO et al., 2009;

BENVENUTTI et al., 2009).

For chemical composition analysis pasture samples (grazing horizon 15 to 25

and 15 to 35 cm) were cut and collected using a rectangle frame (0.5 m2), from 3

distinct points prior to animals’ entry in every paddocks for every grazing cycle. Every

batch of ground corn was sampled. Feed samples were oven dried at 55oC for 72

hours and ground through a 1 mm screen (Wiley mill). Pasture samples were bulked

as one composite sample by treatment. Corn samples were bulked as one composite

sample. Residual moisture content was determined by drying samples at 105ºC for

24 h. Organic matter content of samples was determined after incineration at 550ºC

for 8 h in a muffle furnace (ASSOCIATION OF OFFICIAL ANALYTICAL CHEMISTS -

AOAC, 1990). Nitrogen content was determined by the Dumas combustion method

(Leco 2000, Leco Instruments Inc) and a conversion factor of 6.25 was used to

convert total nitrogen to CP. Contents of NDF, ADF and lignin were determined

according to Van Soest et al. (1991) with no sulphite for all samples analyses and the

addition of amylase for corn grain analyses only. The NDIN and ADIN were

determined using micro Kjeldahl (AOAC, 1990) after digestion of samples with

neutral (no sulphite) and acid detergents (VAN SOEST et al., 1991). The TDN values

of feed samples were calculated according to Weiss et al. (1992). Forage samples

were also analyzed for soluble nitrogen (KRISHNAMOORTY et al., 1982) and for

non-protein nitrogen (NPN) (LICITRA et al., 1996) to estimate the protein fractions

according to CNCPS (SNIFFEN et al, 1992).

3.2.4 Rumen Fluid and Blood Samples

Samples of rumen fluid were collected to determine N-NH3 and VFA

concentrations. On day 16 of every experimental period rumen fluid samples

(approximately 100 mL) were collected at 0, 2, 4, 6, and 8 h after supplement feeding

with the use of a sampling probe (DANÉS et al., 2013). The rumen pH was measured

immediately and samples were frozen at -20°C for further analyses. Rumen fluid

samples were thawed at room temperature centrifuged (15,000 g, 4ºC, 30 min) and

47

analyzed for VFA using gas chromatography (PALMIQUIST; CONRAD, 1971), and

for N-NH3 by phenol-hypochlorite procedure (CHANEY; MARBACH, 1962).

Blood samples were collected on day 15 of the experimental period 4 h after

feedin g, from the coccygeal vessels of each steer directly into a lithium heparin

coated vacutainer [Vacuette, Greiner Bio-One (Americana, SP, Brazil)]. The

vacutainers were inverted six to eight times and placed on ice. Plasma was collected

after centrifugation (3,000 g, 4º C, 20 min) and stored frozen at -20°C for further

analyses. Levels of plasma glucose were analyzed by automatic biochemical

analyzer YSI 2700 Select.

3.2.5 Forage Intake and digestibility

Forage intake was calculated from total fecal excretion and the digestibility of

feeds. On day 1 and for 13 consecutive days, chromium oxide (Cr2O3), previously

weighed in two paper capsules (5 g each), was dosed into the rumen to all steers,

one dose right after supplement feeding in the morning (1000 h) and a second dose

in the afternoon (1700 h). Fecal grab samples (approximately 200 g per animal) were

collected between days 9 to 13, twice a day (1000 h and 1700 h), dried at 55°C for

72 h and ground through a 1-mm screen (Wiley mill). Equal amounts of fecal DM

were mixed to obtain one representative sample for each steer for every

experimental period. The digestibility of feeds (pasture and supplement) was

estimated using the indigestible neutral detergent fiber (iNDF) content as an internal

marker (CASALI et al., 2008). The CP and NDF concentrations were analyzed in the

feces and multiplied by fecal production, to calculate CP and NDF digestibility.

For analyses of fecal chromium concentration, approximately 0.3 g of sample

was digested with 6 mL nitric acid and 2 mL perchloric acid and then made up to 30

mL with RO water (VEGA; POPPI, 1997). The digested samples were analyzed using

an inductively coupled plasma spectrometer (ICP-OES).

Forage intake was calculated dividing forage fecal excretion (total fecal excretion

minus the amount resultant from the supplement) by the forage indigestibility.

3.2.6 Animal Behavior

Grazing behavior measurements took place on d 14 of each period. Four

trained observers were assigned to visually determine grazing (eating plus

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searching), rumination, and idling time every 5 min for consecutive 24 hours The

number of observations for each activity during a 24 hours period was multiplied by 5

minutes to get the daily amount of time spent in that activity. While grazing, cattle

search, acquire into the mouth, masticate, and swallow herbage (GIBB, 1998). The

observers also counted bite rate (bites/min) using the time spent to 20 bites

(PENNING; RUTTER, 2004).

Each eating step was considered a feeding station (RUYLE; DWYER, 1985;

ROOK et al., 2004). The feeding stations per minute were calculated considering the

time spent to 10 feeding station divided by 10. Also, it was measured the number of

steps between feeding stations, and then it was calculated the total steps per day,

multiplying the steps per minute by grazing time (min).

3.2.7 Microbial Protein Synthesis and Nitrogen Efficiency

Nitrogen intake was calculated by the sum of intakes of N from forage and

supplement origins. Apparent efficiency of N use (assuming no retention or

mobilization of N in the body) was calculated for every steer by dividing mean

retained N (N intake minus N in urine and feces) by the average N intake.

Microbial synthesis was estimated through the excretion of purine derivatives

(STANGASSINGER et al., 1995). Spot urine samples were collected on day 15 of

each experimental period, in a single sample collected approximately 4 h after

supplement feeding. Sub-samples of urine (10mL) were acidified (pH<3.0) using 40

mL of 0.036 N H2SO4 and stored at −20°C (VALADARES et al., 1999). Urine samples

were analyzed for creatinine, allantoin and uric acid using a HPLC (PIMPA;

BALCELLS, 2002), and for N using micro Kjeldhal (AOAC, 1990).

Total urine production was estimated using the creatinine content in the

sample, assuming that excretion of creatinine is constant (0.213 mmol kg BW -1)

(CHIZZOTTI et al, 2008). The excretion of purine derivatives (PD) was then

calculated as the sum of allantoin and uric acid excreted in urine, expressed in

mmol/d. The analyses of PD were performed according to Pimpa et al. (2001) using

values of endogenous contribution of PD for zebu cattle, and the intestinal flow of

microbial nitrogen compounds was calculated on the basis of microbial purines

absorbed as described by Chen and Gomes (1992).

49

3.2.8 Rumen Fermentation Kinetics

The in vitro degradation rate of pasture sample was estimated using the semi-

automated method of cumulative gas production adapted by Mauricio et al. (1999).

Four replicates of approximately 1 g of feed samples of 2 pre-grazing sward height

(25 and 35 cm) were incubated using 10 ml of rumen inoculum obtained from a steer

grazing Marandu palisadegrass and fed 1 kg/d of supplement containing ground corn,

soybean meal, urea and minerals, mixed with 90 ml of batch culture.

The pressure of produced gases was measured at 1, 2, 3, 4, 6, 8, 10, 12, 14,

17, 20, 24, 28, 36, 48, 72, 96 and 120 hours after incubation using a pressure gauge

transducer (PDL800). The value obtained from each reading was then subtracted

from a standard value (sample with no substrate). The bicompartmental logistic

model proposed by Pell and Schofield (1993) was used to predict the final volume of

gas from the degradation of fibrous carbohydrates (VFC), the total gas production

from the degradation of non-fibrous carbohydrates (VNFC), lag time (L), and the

degradation rate of fibrous and non-fibrous carbohydrates (KdFC and KdNFC,

respectively).

3.2.9 Statistical Analysis

The experimental design was a double 4x4 Latin Square design, with eight

animals, four treatments and four periods. All variables, except the rumen

fermentation kinetics and its predicted parameters, were analyzed using the PROC

MIXED package of SAS statistical system (SAS® version 9.2©, 2008). The rumen

fermentation kinetics variables were analyzed in a completely randomized design

using PROC GLM package of SAS statistical system (SAS® version 9.2©, 2008). The

model included treatment, animal and period as sources of variation to analyze the

data of forage, nutrient intake, animal behavior, N excretion, blood parameters and

microbial synthesis. Treatments and periods were considered fixed, while animal and

overall errors were considered random effects. The model used to analyze rumen

parameters (pH and concentrations of VFA and NH3-N) collected at different times

after feeding, included treatment, period, animal, hours post-feeding as repeated

measures and treatment x hours post-feeding interaction as sources of variation.

All variables were considered fixed, except animal, whole-plot error, and

subplot error, which were considered random. The covariance structure used for

50

repeated measures analyses was chosen based on Bayesian Information Criteria

parameter of SAS statistical system (SAS® version 9.2©, 2008) closer to zero.

The averages were calculated by Least squares means and adjusted for

comparison by Tukey test at 5% and 10% probability. Thus, significance was

declared at P<0.05 and P<0.01.

The parameters of rumen fermentation kinetics predicted by the equations

were fitted by nonlinear models using PROC NLIN of the SAS statistical system

(SAS® version 9.2©, 2008). Necessarily, all data sets were tested before the final

overall analysis to ensure that all the assumptions of analysis of variance (additive

model, independence of errors, data normality and homoscedasticity) were met.

3.3 Results

3.3.1 Pasture and Supplement Samples

The forage CP, NDF and TDN values for 25 cm grazing management were

13.8, 58.8 and 57.3%, and for 35 cm were 11.0, 63.4 and 54.9%, respectively (Table

7). Even though the CP content for 35 cm had been slightly lower than for 25 cm, the

A, B and C fractions were similar between 25 and 35 pre-grazing sward height

(Fraction A: 27.5 and 26.7 % CP, Fraction B: 63.5 and 64.0, Fraction C: 8.9 and 9.2,

respectively) (Table 8). The energy supplement contained 9.3% CP and 87.3% TDN.

Table 7 - Composition of forage (grazing horizon samples) and ground corn

Pre-grazing sward height Ground corn

25 cm 35 cm

Ash, % DM 9.97 9.34 0.90

CP, % DM 13.87 11.02 9.35

EE, % DM 1.18 0.93 3.67

NDF, % DM 58.80 63.40 11.17

ADF, % DM 33.05 35.85 2.93

HEM, % DM 25.76 27.55 8.24

CEL, % DM 29.96 32.44 2.39

Lignin, % DM 3.29 3.40 0.54

TDN, % DM 57.33 54.89 87.34

1Total digestible nutrients (WEISS et al., 1992)

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Table 8 - Carbohydrates and protein fractions of forage (grazing horizon samples)

Protein

Fractions

Pre-grazing sward

height, cm Carbohydrates

Fractions

Pre-grazing sward

height, cm

25 35 25 35

A, % CP 27.57 26.72 CHOT1, % DM 74.99 78.71

B1, % CP 12.65 7.59 A+B1, % CHOT 10.73 6.91

B2, % CP 37.84 49.61 B2, % CHOT 78.39 79.79

B3, % CP 13.02 6.85 C, % CHOT 10.88 13.29

C, % CP 8.93 9.24

3.3.2 Grazing Behavior and Feed Intake

There was no interaction between grazing management and level of energy

supplementation (P>0.05) for grazing behavior (Table 9) and feed intake (Table 10).

Animals grazing pastures managed with 25 cm sward height spent less time on

grazing activity and spent more time resting and had greater bite rates (P<0.05)

compared to cattle grazing pasture with 35 cm sward height as the start grazing point

(Table 9).

The number of feeding stations per minute was greater (P<0.05) for animals

grazing pastures with 25 cm, but with less steps between feeding stations. The steps

per minutes were not affected by grazing management (P>0.05). However, as

consequence of longer grazing time for the 35 cm treatment the total steps per day

also was greater for 35 cm pre-grazing sward height, given the fact that steps/min did

not change (P>0.05).

Energy supplementation decreased (P<0.05) grazing time but had no effects

on the other grazing behavior variables (P>0.05).

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Table 9 - Effects of levels of energy supplementation and grazing management on

grazing behavior

Management,

Cm

Level of

supplementation,

% BW

P value SEM1

25 35 0 0.3 M S M*S

Grazing time 386.88 465.25 445.94 406.18 0.0001 0.0236 0.6380 20.29

Ruminating time 385.00 384.49 379.99 389.50 0.9767 0.5862 0.4167 19.73

Rest time 608.12 530.73 554.20 584.66 0.0005 0.1199 0.7240 18.33

Bite rate, bit/min 34.23 22.78 28.20 28.80 0.0003 0.8253 0.2945 3.28

Feeding

stations/min 5.06 4.13 4.61 4.58 0.0145 0.9296 0.9849 0.35

Steps between

feeding stations 1.29 1.60 1.52 1.38 0.0157 0.2457 0.2275 0.11

Steps/min 6.51 6.58 6.90 6.18 0.9201 0.2902 0.3446 0.49

Steps/day 2524.0 3009.8 2922.7 2611.0 0.0973 0.2803 0.6364 215.8

1Standard error of the means, M=management, S=supplementation, M*S=interactions between

management and supplementation

Cattle grazing pastures managed with 25 cm (correlated to 95% LI) had

greater intakes (P<0.05) of forage DM, total DM, digestible DM, NDF and CP than

animals grazing pastures managed with 35 cm as a start grazing point (Table 10).

Supplementing cattle with ground corn at 0.3% BW reduced forage DMI

(P<0.05) and NDF intake (P<0.10), with no effect (P>0.05) on intakes of total DM,

digestible DM, and CP. Feeding supplement at 0.3% BW resulted in a substitution

rate of 1.63.

53


intakes of forage DM, total DM, digestible DM, NDF and CP and on

substitution rate

Management,

cm

Level of

supplementation,

% BW

P value SEM1

25 35 0 0.3 M S M*S

Forage DMI, % BW 1.86 1.32 1.79 1.38 0.0104 0.0454 0.9905 0.14

Total DMI, % BW 2.01 1.47 1.79 1.68 0.0101 0.5788 0.9905 0.14

Digestible DMI, % BW 1.41 1.00 1.20 1.21 0.0094 0.9823 0.9347 0.08

NDF DMI, % BW 1.11 0.85 1.09 0.88 0.0381 0.0872 0.918 0.09

CP DMI, % BW 0.27 0.15 0.22 0.20 0.0001 0.3443 0.8105 0.01

Substitution rate2 0.00 0.00 0.00 1.63 0.9652 0.0019 0.8107 0.25

Forage DMI, kg/d 6.34 4.56 6.19 4.72 0.0015 0.0295 0.8307 0.53

Total DMI, kg/d 6.85 5.08 6.19 5.75 0.0096 0.4887 0.8241 0.54

Digestible DMI, kg/d 4.81 3.49 4.16 4.14 0.0041 0.9519 0.8985 0.35

NDF DMI, kg/d 3.79 2.94 3.76 2.97 0.0414 0.0542 0.7642 0.32

CP DMI, kg/d 0.92 0.55 0.78 0.69 0.0001 0.2857 0.9659 0.07

1Standard error of the means,

2= kg of reduction on forage DMI for kg of supplemented fed,

M=management, S=supplementation, M*S=interactions between management and supplementation

3.3.3 Apparent digestibility

The forage and diet DM and CP digestibility were greater (P<0.05) for 25 cm

pre-grazing sward height than for 35 cm (Table 11). Energy supplementation

increased (P<0.05) diet DM digestibility (P<0.05), with no negative effect (P>0.05) on

NDF digestibility.

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Table 11 - Effects of energy supplementation and grazing management on forage

and diet total tract digestibility

Management,

cm

Level of

supplementation,

% BW

P value SEM1

25 35 0 0.3 M S M*S

Forage digestibility (%)

DM 68.13 65.44 67.31 66.26 0.0013 0.1683 0.6518 0.78

CP 74.68 63.87 70.17 68.37 0.0001 0.1846 0.8509 0.94

NDF 63.42 62.57 62.44 63.55 0.5189 0.3946 0.5556 1.19

Diet digestibility (%)

DM 70.13 68.43 67.31 71.25 0.0523 0.0001 0.1213 0.97

CP 75.85 66.55 70.20 72.21 0.0001 0.1391 0.1764 1.36

NDF 63.12 62.13 62.44 62.80 0.4458 0.7786 0.6332 1.16



3.3.4 Rumen Fluid and Plasma Glucose

There was no interaction between grazing management and energy

supplementation (P>0.05) for both rumen parameters and plasma glucose (Table

12).

The rumen pH was not affected by energy supplementation (P>0.05), but it

was decreased by the 25 cm grazing management (P<0.05). The rumen N-NH3 was

greater (P<0.05) for cattle grazing the pasture managed with 25 cm height as a start

grazing point compared with the 35 cm height. Feeding ground corn to grazing cattle

decreased (P<0.05) rumen N-NH3. Total VFA concentration was not affected

(P>0.05) by treatments but supplementing energy decreased (P<0.05) rumen

acetate, increased (P<0.05) rumen propionate and decreased (P<0.05)

acetate:propionate ratio. Plasma glucose was not affected by increasing energy

supplementation and grazing management (P>0.05).

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Table 12 - Effects of energy supplementation and grazing management on rumen

pH, N-NH3, volatile fatty acids (VFA) and plasma glucose

Management,

cm

Level of

supplementation,

% BW

P value SEM1

25 35 0 0.3 M S M*S

pH 6.39 6.52 6.46 6.44 0.0015 0.5734 0.3432 0.14

NH3/N,

mg/dL 11.22 9.77 11.28 9.70 0.0210 0.0120 0.3397 1.10

Total

VFA,mM 145.09 145.55 144.40 146.04 0.3203 0.8211 0.7757 7.05

VFA mol/100 mol

Acetate 69.84 70.54 71.09 69.37 0.7026 0.0063 0.9879 0.90

Propionate 19.35 19.23 18.49 20.24 0.9516 0.0001 0.5198 0.53

Butyrate 10.98 10.27 10.42 10.70 0.2281 0.5715 0.9620 0.63

A:P 3.75 3.74 3.95 3.51 0.7488 0.0001 0.4043 0.13

Plasma

glucose 51.77 53.21 54.30 53.89 0.9821 0.9590 0.6532 1.89



3.3.5 Microbial Protein Synthesis and Nitrogen Use

There was no interaction (P>0.05) between energy supplementation and

grazing management for nitrogen use and microbial synthesis (Table 13). Feeding

only 0.3% BW of ground corn did not improve (P>0.05) the use of N by cattle grazing

fertilized Palisadegrass pasture. Microbial synthesis and efficiency (P>0.05) (Table

13). However, managing the pasture with 25 cm pre-grazing sward height caused a

huge increase (P<0.05) in the intake of N with greater retention of N (P<0.05) and

greater efficiency of N utilization (P<0.05) by cattle grazing fertilized Palisadegrass

pasture.

56


nitrogen intake, nitrogen urine, nitrogen feces, nitrogen retained (% N

intake), microbial protein (MicP) and microbial efficiency (MicEf)

Management,

cm

Level of

supplementation,

% BW

P value SEM1

25 35 0 0.3 M S M*S

N fecal2 35.34 34.86 36.9 33.29 0.9392 0.562 0.7578 8.8

N urinary2 41.99 44.62 44.73 41.89 0.6336 0.6336 0.7898 8.35

N excretion2 77.48 79.49 81.64 75.33 0.771 0.3653 0.6275 16.1

N intake2 155 100.41 130.2 125.21 0.0064 0.7855 0.892 24.11

N retention2 77.36 20.91 48.55 49.25 0.0089 0.9715 0.99 21.08

N retention,

%N intake) 49.7 20.82 39.9 40.68 0.0446 0.9296 0.6626 6.66

MicP2 457.07 411.25 425.01 443.32 0.3114 0.6830 0.6163 62.35

MicEf.

g/kg of TDN 111.97 145.63 130.29 127.30 0.0935 0.8776 0.4861 14.48


2g/day, M=management, S=supplementation, M*S=interactions

between management and supplementation


Forage samples from pastures managed with 25 cm height as a start grazing

point presented shorter (P<0.05) lag time and greater (P<0.05) degradation rate of

fibrous carbohydrate (Table 14) compared with forage from the 35 cm pastures.

57

Table 14 - Effect of pasture management on rumen kinetics fermentations

parameters of Palisadegrass

Parameters* Pre-grazing sward height

P value SEM 25 cm 35 cm

fVFC, mL/g DM 121.4 119.3 0.5887 2.7

kdFC, %/h 0.0200 0.0182 0.0371 0.0008

fVNFC, mg/ g DM 107.3 100.8 0.3090 3.4

kdNFC, %/h 0.0718 0.0735 0.4842 0.0041

Lag time, h 5.4 6.0 0.0153 0.2

SEM= standard error of the means; fVCF= final volume of fibrous carbohydrate; kdCF= rate of degradation

of fibrous carbohydrate; fVNFC= final volume of non-fibrous carbohydrate; kdNFC= rate of digestion of

non-fibrous carbohydrate.*Parameters estimated according to Pell and Schofield, 1993

3.4 Discussion

3.4.1 Pasture and supplement samples

Many factors may affect the forage’s nutritive value such as its physiological

stage, the soil organic matter content and nitrogen applied as fertilizer (TAIZ;

ZEIGER, 2004) altering especially the CP content (JONHSON et al., 2001; DANÉS et

al., 2013). Research data, from studies using cattle grazing tropical grasses during

the wet season reported NDF content varying from 58 up to 75% and CP content

from 5 up to 22% (MORAIS et al., 2009; DANÉS et al., 2013; FLORES et al., 2008).

The forage contents of NDF and lignin were lower and the CP contents were

higher than usually found in tropical grasses (TEDESCHI et al., 2002; MORAIS et al.,

2009; VIEIRA et al., 2000; NASCIMENTO et al., 2009) as a consequence of intensive

grazing management, N fertilization, and the sampling method (i.e., the part of the

plant that was sampled).

When the pastures are managed to be grazed at their optimal point (meaning

variable grazing intervals), compared with fixed intervals, which for tropical grasses

are traditionally longer than the average of the variable ones, not only is the forage

offered to the animal of better quality, but the sward structure also increases the

grazing efficiency (CARNEVALLI et al., 2006). In this present study the CP, NDF,

Lignin and TDN values for 25 cm pre-grazing sward height were 13.8, 58.8, 3.3 and

57.3%, respectively. For 35 cm of sward height these nutrients values were 11.0,

58

63.4, 3.4 and 54.8, respectively (Table 7). Both 25 and 35 cm pastures may be

considered with medium nutritive value according to NRC (1996, 2001) standard

values for high quality forage.

Recently, Lopes (2011) characterized the nutrient profile and fiber digestibility

of the main tropical grasses produced under intensive rotational grazing management

in Brazil. One-hundred six samples of Palisade grass (Brachiaria brizantha); Mulato

grass (Brachiaria hibrida), Bermuda grass (Cynodon dactylon); African Bermuda

grass (Cynodon nlemfuensis), Guinea grass (Panicum maximum) and Elephant

grass (Penisetum purpureum) were evaluated. Tropical grasses presented 14 to 21%

CP and 60 to 63 % NDF, averaged by grass species. Comparison between mean of

in vitro NDF digestibility estimates of tropical grasses by time-point and alfalfa silage

standard indicates that tropical grasses had greater fiber digestibility than the alfalfa

silage. The author concluded that tropical forages subjected to intensive grazing

management practices can be relatively high in crude protein content and in fiber

digestibility.

Despite the lower IVNDFD, alfalfa presents lower NDF content with greater

particle fragility. Allen (2000) and Kammes and Allen (2012) reported that lactating

cows presented greater DMI with diets containing alfalfa silage compared to grass

silage in spite of greater NDF digestibility for grass silage. The explanation was that

the filling effect of alfalfa was less because of greater particle fragility, which

decreased retention time in the rumen reticulum and resulted in less distention and

greater DMI.

Cattles grazing only tropical grass are usually under imbalance between

protein and fermentable carbohydrate (NRC, 1996; DANÉS et al., 2013). In well

managed tropical grass this fact may be intensified, because the CP content

increases, but the NDF does not decrease in order to provide improvements in

microbial synthesis as resulted of more available energy into the rumen (SANTOS et

al., 2014). Thus, in the current study is possible to observe that for the 25 cm grazing

management the CP content was 13.8% with high A+B1 (40.22% of CP) fractions,

which present fast degradation rates. However, the NDF content was 58.8%, with low

A+B1 carbohydrate fractions (10.73% of CHO) what may suggest imbalance

between protein and energy, and low efficiency on nutrient usage, mainly CP. The

same situation happened for forage from the 35 cm pastures (Table 8).

59

According to NRC (1996), tropical grasses containing 12% CP or more, with

71% of that CP as rumen degradable protein (RDP), and up to 65% TDN, supply

protein in excess (RDP and metabolizable protein) but limit the performance of

yearling growing cattle because of energy shortage. The low content of carbohydrate

A + B1 fraction associated with high levels of CP, high in RDP, in well managed

marandu palisadegrass may result in considerable losses of protein in the form of N-

NH3 from the rumen (NRC, 1996). The higher is the level of N fertilization, the higher

is the forage CP content (JONHSON et al., 1991), what may result in an increased

imbalance between fermentable carbohydrate and RDP (RUSSELL et al., 1992).

Supplementing rumen fermentable energy may be an effective tool to improve

performance of grazing cattle and improve the utilization of forage protein.

3.4.2 Animal Behavior and Forage Intake

The rumen fill effect, caused by high fiber content in grasses, has been

pointed out as the main factor that limits the forage intake in grazing animals

(STOBBS, 1973; POPPI et al., 1980, 1981a, 1981b, 1985). However, grazing

animals live the daily challenge of harvesting their own feed, which is often limited by

structural characteristics of the sward (HODGSON, 1977; ILLIUS; GORDON, 1987;

CANGIANO et al., 2002). The sward structure starts limiting DMI even before the

chemical composition of forage plays its role. Trindade (2007) evaluated the 95% and

100% LI as the start grazing point criteria for Palisadegrass (Marandu), which

resulted in average sward height of approximately 25 cm and 35 cm, same as the

used in this trial. The combination of 25 cm premise with 15 cm post-grazing height

resulted in greater proportions of leaves and less proportions of stem and dead

material in the forage available to animals, compared to pastures managed with

100% LI (it would be 35 cm pre-grazing sward height) and 15 cm, respectively, what

corroborate with the morphological composition presented in this current study, as

showed in Table 6.

The increases on sward height from 25 to 35 cm decreased DMI (total, forage,

digestible, NDF and CP, Table 10) as response of decreases on harvesting efficiency

(Table 9). This reduction in efficiency during the grazing down process is the result of

greater proportion of stems that may acts as a horizontal barrier causing reductions

on bite rate and bite area (UNGAR et al., 2001; BENVENUTTI et al., 2006;

60

DRESCHER et al., 2006; BAGGIO et al., 2009; BENVENUTTI et al., 2009) as

observed for the bite rate values at 25 cm (34.23 bites/min) and 35 cm (22.78

bites/min) grazing managements (Table 9). Baggio et al. (2009) also reported

reductions in bite rates as sward height increased. According to Carvalho (1997) the

forage mass is proportional inversely a bite rate, once that in pastures with highest

sward height and bite mass, the animal spend more time manipulating and chewing.

This fact results in more time spent for each bite interval, what reduce the bite rate.

In the mentioned study conducted by Trindade (2007) it was reported greater

proportions of leaves throughout the grazing down process in the extrusa of animals

from 25 cm treatment, while animals grazing paddocks managed using 100% LI (or

35 cm) as the pre-grazing starting point had a greater participation of stems and dead

material in their extrusa. Trindade (2007) results are in agreement with founds in this

current study.

Animals grazing pastures managed at 25 cm had 78 minutes less for grazing

time and 77 minutes more for resting time (Table 9). The difficult caused by stems

during the grazing down process leaded the animals to increase the grazing time,

such as happened for 35 cm treatments. However, even with possible

compensations in bite mass, animals grazing pastures managed at 35 cm of sward

height were not able to increase their DMI in comparison with 25 cm treatment.

The combinations between high bite rates and low grazing time suggest

greater harvesting efficiency, what may be confirmed with greater DMI (Table 10) for

animals grazing pastures with 25 cm of sward height. Also, this information suggest

less energy expenditure for grazing activities for 25 cm treatments, supported by all

mentioned parameters, mainly by greater resting time. Gimenes et al. (2011)

evaluated the animal performance of Nellore bulls grazing Palisadegrass managed at

the same pre-grazing sward height (25 vs 35 cm) during one year and they found the

ADG 120 g more for animals that grazed pastures managed at 25 cm of sward

height. Also, they found differences in stocking rate between 25 and 35 cm (3.13 and

2.85 AU/ha, respectively) and live weight production (886 and 674 kg/ha,

respectively). The authors attributed the greatest response with 25 cm of sward

height for greater harvesting efficiency, pointing the importance of grazing

management on DMI and animal performance.

Two previous studies conducted at ESALQ/USP facilities found higher milk

yields when cows were managed under height-based variables than when they were

61

managed under fixed grazing intervals (CARARETO, 2007; VOLTOLINI et al., 2010).

The improvements on milk yield are attributed to greatest harvesting efficiency, once

that the forage nutritive value were similar between grazing managements. In

addition, the pre-grazing sward height average was higher for fixed grazing intervals

management than for height-based (or LI concepts).

Animals grazing pastures managed at 35 cm spent more time per feeding

station, resulting in less feeding station per min (Table 9), what can be related with

more forage mass and the time spent to manipulating it in each feeding station

(BAGGIO et al., 2009). The number of steps between feeding station and the total

steps per day found for 35 cm treatments showed that animals walked more

searching for better feeding stations. Feeding station is defined as the area of

pasture a grazing animal can reach at each eating step (SEARLE et al., 2005). The

forage allowance at each feed station determines the time spent by the animal at the

feed station (BAGGIO et al., 2009). Others authors also characterize this feeding

station usage as feeding station profitability (WADE; GREGORINI, 2010,

GREGORINI et al., 2011). Thus, animals start grazing at one feed stations, keeping it

there until the forage allowance starts to get reduced, after that animals give up to

keeping harvest and choose another feeding station (SEARLE et al., 2005). The

distance walked between feeding stations may indicate how difficult is to find another

new feeding station.

There was no effect of energy supplementation on feeding station/min, steps

between feeding stations, steps/min and steps/day. In contrast, some authors found

effects of supplementation on displacement patterns, where non-supplemented beef

heifers traveled greater distances and visited more feeding stations per minute, while

the opposite happened for supplemented animals (GLIENKE et al., 2010). A

hypothesis raised was that animals spent less time searching for feeding stations

because they had their nutritional requirements met. In the other hand, other study

reported that supplemented animals were more selective and covered greater

distances in search for better feeding stations (ADAMS, 1985).

The energy supplementation fed at 0.3% BW affected forage DMI for both 25

and 35 cm grazing managements, without interactions (grazing

management*supplementation) (Table 10) Thus, the energy supplementation

decreased just the forage DMI as consequence of substitution effect, without

increments in total and digestible DMI.

62

The intake regulation mechanism may be altered when supplements are used,

because at certain levels of supplementation, the caloric density of the diet is

expected to increase and control DMI through physiological effect mechanisms

(MOORE et al., 1999). The physiological control mechanism apparently did not take

effect at the used levels of supplementation (0.3% BW) in this study, since total DMI

was not decreased. However, the lack of increases on digestible DMI may result in

negative effects because one of the most important objectives to use energy

supplementation is to increase the energy DMI and then animal performance.

Some theories were discussed in the past regarding the use of supplements to

lead decreases on rumen pH, resulting in DMI reduction (ULMER et al., 1990;

LEVENTINI et al., 1990). These observations does not corroborate with the results of

this study, since corn supplementation caused no significant reductions in rumen pH,

and no negative effects on fiber degradation (Table 11 and Table 12) but in spite of

that, there had been decreases on DMI. Thus, it’s in accordance with the literature

that shows no reduction on ruminal pH below 6.2, but decreases on forage DMI

(CHASE; HIBBERD, 1987; PORDOMINGO et al., 1991; ELIZALDE et al., 1998).

However, other factors, such as hormonal regulation and propionate concentration

(ALLEN et al., 2009), besides pH, acting on DMI reduction may play a more

important role.

One important point in this study is related with the grazing method (rotational

grazing system) used, that could have increased the substitution rates during the end

grazing period, promoted by reductions on forage allowance along abasement. In

studies conducted using another grazing method (continuous grazing system) the

response on substitution rate when 0.3% BW was fed was different (CASAGRANDE,

2010; VIEIRA, 2011), suggesting that animals suffer a great effect of low levels of

energy supplementation on ingestive behavior during the grazing process, mainly

when submitted at the end of grazing period.

Higher rates of substitution for low levels of energy supplementation were

reported by Lake et al. (1974), Pordomingo et al. (1991) and Hess et al. (1996), all

using corn. In the work of Lake et al. (1974), with cattle grazing orchardgrass with 17

to 20% CP content, it was observed a decrease on forage intake from 2.89% down to

2.57% BW when corn was offered at level of 0.44% BW. Pordomingo et al. (1991)

reported decrease on forage intake from 2.76 down to 2.26% BW for grazing animals

on temperate grasses with 10.6% CP, and supplemented with whole corn at 0.4%

63

BW. In the study of Hess et al. (1996) animals grazing fescue with 14.4% CP, had a

decrease on forage intake from 3.49 down to 2.82% BW when they were fed cracked

corn at 0.34% BW.

Energy supplementation levels around 0.3% BW are more frequent in

Brazilian cattle production systems for growing beef cattle on pastures during the wet

season, than greater levels, such as 0.6 or 0.9% BW. The high substitution rate

observed in this study indicate that low energy supplementation levels such as 0.3%

BW, during the wet season, present greater economic risks than higher levels of

supplementation. However, in disagreement with the present work, Brokaw et al.

(2001), observed a small decrease on forage intake (2.00 vs. 1.92% BW) when

animals grazing a 17% CP bromegrass (Bromus inermis), were supplemented with

cracked corn at 0.34% BW.

According to Bailey (1961), the increases on forage DMI observed for non-

supplemented treatments should result in longer rumination time, however, it was not

observed in that study as well as in the presented data (Table 9).


Considering that energy supplement (corn) can provide a potential substrate

for propionate production, some authors (MARTIN; HIBBERD, 1990; FALKNER et

al., 1994) attributed increases in concentration of propionate and changes in acetate:

propionate ratio, to use of supplements. This fact is corroborated by the current

study, in which it was observed a significant decrease (P<0.05) for both molar

proportion of acetate and acetate: propionate ratio, and increases on propionate

molar proportion when 0.3% BW of supplement was fed (Table 12).

Total VFA production, molar proportions of butyrate and ruminal pH were not

affected by energy supplementation (P> 0.05). However, the ruminal pH was affected

by grazing managements, being less for 25 cm (6.39) than for 35 cm (6.52) (Table

12) with average values remaining above 6.2. According to Orskov (1982) and

Mertens (1977), microbial growth is not affected until pH drops below 6.2. The results

found in this work are corroborated with some reports published in the literature

(CATON; DHUYVETTER, 1997; SANTOS et al., 2009) where is emphasized that

there is lack of reliable data pointing to rumen pH as the cause of reductions on fiber

digestibility, and attributed effects on DMI to other factors.

64

The N-NH3 in the rumen fluid of steers in this study decreased linearly

(P<0.05) with supplementation (0.3% BW) and grazing management (25 cm pre-

grazing sward height) (Table 12). A possible explanation for the supplementation

effect is that a higher intake of readily fermentable carbohydrates found in the

supplement would provide more substrate for microbial synthesis.

As already mentioned, starch fermentation may influence the rate of utilization

of rumen N-NH3 by changing the energy supply for microbial growth (RUSSELL;

MARTIN, 1984). Studies indicate that microbial synthesis is maximized when starch

fermentation and proteins degradation are synchronized (FIRKINS, 1996; RUSSELL

et al., 1992; BACH et al., 2005). Satter and Slyter (1974) reported that concentrations

between 2 and 5 mg N-NH3/dL of rumen fluid were required to allow maximum

microbial growth. Sampaio et al. (2010) reported 6.24 mg/dL to maximize NDF

degradation and Lazzarini et al. (2009) found 15.33 mg/dL to maximize DMI in

animals consuming tropical grass (Brachiaria decumbens).

Non-supplemented animals had rumen concentration of N-NH3 in the current

study of 11.28 mg/dL. Studies conducted by Detmann et al., 2001, Sales et al.,

2008b, Sales et al., 2008a, with tropical grass (Brachiaria decumbens) averaging

9.88, 8.75 and 8.97% CP content, respectively, reported levels of N-NH3 in rumen

fluid of 9.88, 11.89 and 10.68 mg/dL, respectively. Franco et al. (2002) with marandu

palisadegrass, averaging 10% CP, reported level of N-NH3 in rumen fluid of 7.3

mg/dL.

The N-NH3 concentrations were 11.22 and 9.77 mg/dL for 25 and 35 cm pre-

grazing sward height (Table 12) and the A+B1 protein fractions were 44.22 and

34.31% (CP basis), respectively (Table 8). It suggests that CP levels and more

specifically A+B1 protein fraction may alter the rumen N-NH3 concentration.

Experimental treatments had no effects on blood glucose (P>0.05). Reed et al.

(2007) reported no effects of protein supplementation for steers receiving low-quality

hay. Hersom et al. (2004) also did not found differences on blood glucose between

non-supplemented and energy supplemented steers.


The energy supplementation did not alter (P>0.05) the nitrogen use, when

0.3% of fine ground corn was fed (Table 13). The values of urinary excretion

65

observed for non-supplemented and supplemented animals was lower (44.73 and

41.89 g/d, respectively) than found in the literature by Nascimento et al. (2010) that

worked with Brachiaria decumbens and energy-protein supplementation fed at 0.4%

BW. The non-supplemented animals used by the latter author consumed

approximately 0.894 kg of CP/day and excreted 59.4 g/d of urinary nitrogen.

However, the energy-protein supplement (34% of CP) used increased the CP intake

to 1.43 kg, what promoted urinary nitrogen excretion of 128.8 g/d. These excretions

represent 66 and 56% of nitrogen intake for non-supplemented and supplemented

animals (NASCIMENTO et al., 2010), compared to urinary nitrogen excretion at 34

and 33% for non-supplemented and supplemented beef steers in the present study.

It was expected improvements in nitrogen usage and microbial synthesis

regarding energy supplementation, most likely because rumen microbes would be

able to utilize better the nitrogen present in the forage due to greater quantity of

readily fermentable carbohydrates in the rumen (RUSSELL et al., 1992; OWENS;

GOETSCH, 1993). However, the high substitution rate, with no increments on

digestible DMI (Table 10), limited the mentioned improvements on ruminal

fermentation. Several authors (LAZZARINI et al., 2009; FIGUEIRAS et al., 2010;

SOUZA et al., 2010) found no increase on microbial synthesis with protein

supplementation in tropical conditions, compared with the control treatment, what

may suggest energy limitation to improve microbial growth.

These authors found an average 404.68 g/d of microbial synthesis, compared

to 434.16 g/d ub the present study. The average estimate of microbial efficiency was

120.5 g microbial CP/kg TDN (LAZZARINI et al., 2009; FIGUEIRAS et al., 2010;

SOUZA et al., 2010), what is close with suggested by Valadares Filho et al. (2010),

and with the presented in the current study (128.8 g microbial CP/kg TDN) (Table

12). This microbial efficiency value is near to recommended values by NRC (1996)

and Valadares Filho et al. (2006).

Based on NRC (1996), microbial production can vary from 53 to 140 g

microbial CP for each kg of TDN consumed, being used 130 g microbial CP/kg TDN

as microbial efficiency.

Even considering negative effects on microbial synthesis when rumen pH is

below 6.0 due to damages on cellulolytic bacteria growth (RUSSEL; WILSON, 1996;

VERBIC, 2002), in the current study the rumen pH levels observed were above that

66

value and most likely did not affect the synthesis of microbial protein and

consequently NDF digestibility (Table 11 and Table 12).

3.4.5 Apparent digestibility and ruminal kinetics

The supplementation just affected the diet (pasture plus concentrate) DM

digestibility, considering the increase of non-fibrous carbohydrate due to energy

supplementation (Table 11). In contrast with several papers published in the literature

(HANNAH et al., 1989; ZORRILLA-RIOS et al., 1989; VANZANT et al., 1990;

PORDOMINGO et al., 1991) that have found reduction on total tract DM and OM

digestibilities when grain supplementation was fed, in this current study the corn

supplementation at 0.3% BW did not promote reductions on nutrients digestibility,

mainly on pastures NDF. This found corroborate with other several studies (LAKE et

al., 1974; KRYSL et al., 1989; BRANINE; GALYEAN, 1995; CATON; DHUYVETTER,

1997; SANTOS et al., 2009;), which reported improvements or no effect of energy

supplementation on DM, OM or NDF digestibility for cattle consuming forage diets.

The forage protein content may influence strongly the digestibility response to

energy supplementation. In situations in which CP is limiting, energy supplementation

alone theoretically could worsen the CP deficiency and result in reduced intake,

digestibility, and performance (SANSON et al., 1990; CATON; DHUYVETTER,

1997).

However, grazing managements affected diet and pastures nutrients

digestibility, what could be related with the nutritional value found in pastures

managed with 25 cm pre-grazing sward height (Table 7 and Table 8). The increase

on pasture NDF, more expressive on cellulose content (8% of increment) in 35 cm

pastures may explain partially the reduction on DMI. The lag time was shorter for the

25 cm compared to the 35 cm pasture (Table 14), probably it is reflect of the less

NDF content (Table 7) in pastures with 25 cm treatment. According to Mertens e

Loften (1980) the increases in Lag time were responsible to reduce fiber digestion,

since the fiber can pass for the posterior tract. Fiber degradation rate was greater for

25 cm pasture. However, due to very similar composition in terms of fiber quality, it

was not possible to detect huge changes in fiber digestion either by apparent fiber

digestion or by in vitro ruminal kinetics (Table 14). Indeed, pastures managed out of

67

LI criteria (35 cm pre-grazing sward height) have an important difference on sward

structure able to be a limiting factor for forage harvesting.

3.5 Conclusions

There is no interaction between low level of energy supplementation and

grazing management.

Grazing management is more effective to improve energy intake of grazing

cattle than feeding only 0.3% BW of energy supplement.

Grazing management allows cattle not only to increase energy intake but also

to decrease energy expenditure with grazing activity.

Corn supplementation fed at 0.3% BW for cattle maintained in a rotational

grazing systems reduce grazing time and forage intake of steers grazing well

managed (i.e. with adequate levels of CP) Marandu palisadegrass.

Animals supplemented with low levels (0.3% BW) of corn do not have

significant increases in energy intake, what may limit responses in weight gains. The

low supplementation (0.3% BW) strategy may be riskier with no economic return or

even losses because of great substitution effect.

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85

4 FEEDING MEDIUM LEVEL OF ENERGY SUPPLEMET FOR BEEF CATTLE

AND INTERACTIONS WITH GRAZING MANAGEMENT

Abstract

Eight 24-month-old rumen-cannulated Nellore steers (343 kg BW ± 7.40) were used in two 4x4 Latin squares to evaluate the interaction between energy supplementation and pre-grazing sward height on cattle grazing behavior, nutrient intake, digestion and metabolism. Treatments corresponded to 0 (mineral supplementation) or 0.6% (0.6% of BW of ground corn as fed basis) combined with 2 pre-grazing sward heights (25 and 35 cm), with a common stubble height of 15 cm for all treatments. Steers were managed on 2 ha of Palisadegrass pasture (Brachiaria brizantha cv. Marandu) from February to April of 2011. Pastures were fertilized with 120 kg of nitrogen per ha and averaged 13.8 and 11.0% CP and 58.8 and 63.4% NDF, for the 25 and 35 cm grazing treatments, respectively. The parameters evaluated were forage intake, rumen pH and N-NH3, microbial synthesis, VFA, blood glucose, nitrogen balance and grazing behavior. Forage DMI and apparent DM, CP and NDF digestibility were obtained using Cr2O5 and indigestible NDF as digesta markers. Concentration of purine derivative in the urine was used to estimate microbial synthesis. Forage DMI was increased (1.88 vs 1.22%, P<0.05) when pre-grazing sward height was 25 cm and decreased when 0.6% BW supplement was fed (1.77 vs 1.33% BW, P<0.05). The total and digestible DMI were increased by energy supplementation (P<0.05) and when pre-grazing sward height was 25 cm. Steers grazing the 25 cm treatment spent significantly less time grazing and more time resting (P<0.05), took less steps between feeding stations and per day (P<0.05) and presented greater bite rates (P<0.05) when compared with 35 cm treatments. Grazing time was decreased (P<0.05) when supplement was fed, more intensely for animals grazing pastures managed at 35 cm than 25 cm.The DM and CP digestibility of forage and of total diet were greater (P<0.05) for the 25 than the 35 cm grazing management. Energy supplementation increased (P<0.05) diet and forage NDF digestibility and decreased the forage CP digestibility (P<0.05), without effects on CP diet digesbility digestibility (P>0.05). Also, the diet DM digestibility was increased feeding 0.6% BW. (P<0.05). Rumen pH (6.50 vs 6.35) was less (P<0.05) and rumen N-NH3 (12.00 vs 10.01 mg/dL) was greater (P<0.05) for the 25 cm grazing management. Supplementation decreased (P<0.05) rumen N-NH3. Microbial synthesis was increased (P<0.05) by energy supplementation. The nitrogen retention was increased (P<0.05) for the 25 cm grazing management and supplementation. Concentration of total VFA was not affected by treatments (P>0.05). However, the energy supplementation decreased rumen acetate and increased propionate (mol/100 mol), resulting in decreased A:P ratio (P<0.05). Blood glucose was not affected by treatments (P>0.05). The pre-grazing sward height of 25 cm and 0.6% BW of energy supplementation were effective to improve harvesting efficiency and energy intake of cattle grazing tropical pasture under rotational grazing. Keywords: Beef cattle; Energy; Grazing; Management; Supplementation; Tropical

grass

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4.1 Introduction

According to Conrad et al. (1964), forage intake is regulated by the capacity of

the digestive tract to digest low quality forage (physical effect or rumen fill) or by the

feedback caused by nutrients absorbed from high energy diets (physiological effect).

For cattle grazing tropical grasses, once the forage reaches the rumen, the physical

regulation is the major mechanism controlling forage intake. However, the forage

needs to be harvest before to pass into the gastrointestinal tract and the structure of

the sward has a great impact on the efficiency of forage harvesting by grazing cattle,

either for temperate grasses (Hodgson et al., 1994) as for tropical grasses (DA

SILVA; PEDREIRA, 1996; CARVALHO et al., 2001; DA SILVA et al., 2009). Hodgson

et al. (1977) reported that the structure of the sward may have a greater impact on

forage intake of grazing cattle than the physical and phisiological mechanisms

discussed by Conrad et al (1964) and Allen (2000).

Just after the grazing process, the grass initiates its regrowth with intense leaf

appearance and leaf growth and minimal stem elongation. The sward will reach a

determined height where light will start to become limiting for the lower portion of the

sward. Light restriction will stimulate leaves senescence and stem elongation. The

result is an increased proportion of senescent material and stems and a decreased

proportion of green leaves in the sward, with dramatic alteration on the grazing

efficiency and forage intake (DA SILVA; PEDREIRA, 1996; CARVALHO, 2013;

SANTOS et al., 2014).

Even in well managed pastures, where the sward structure can improve the

grazing efficiency and forage intake, the animal performance is still limited for

nutrients intake, what does not permit the expression of the animal’s genetic

potential. The most limitation is caused by energy intake (DANÉS et al., 2013), due to

limitations in forage harvest capacity of the animals and rumen fill caused by fiber

content of the tropical grasses (WILSON, 1994) and low particle fragility of grasses

(ALLEN, 2000).

The use of concentrate supplementation high in energy is a powerful tool to

increase energy intake, stocking rate and cattle performance (VENDRAMINI et al.,

2006, 2007; DA CRUZ et al., 2009; FERNANDES et al., 2010). However an

interaction between grazing managements and concentrate supplementation is not

known. Therefore, the objectives of this study were to evaluate the effects of feeding

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an energy supplement at 0 or 0.6% BW and its interaction with grazing management

on cattle: 1) ingestive behavior; 2) nutrient intake; 3) rumen parameters; 4) blood

glucose; 5) apparent digestibility; 6) rumen fermentation kinetics; and 7) nitrogen use.

4.2 Materials and methods

This article is the second of a set of 2 experiments that evaluated interactions

between grazing management and energy supplementation. This article used a

medium level of energy supplementation (0.6% BW), while in the companion article a

lower level of energy supplementation (0.3% BW) was tested. The experiments had

independent animals, but were carried out simultaneously in the same grazing area.

Thus, the forage chemical and morphological compositions were the same for both

and are presented in the companion article.

4.2.1 Experimental area and animals

The experiment was conducted at the Department of Animal Science of the

University of São Paulo (USP), located in Piracicaba-SP, Brazil, during the months of

February to April of 2011. The experimental area consisted of 2 ha of marandu

palisadegrass managed as a rotational grazing system divided in 16 paddocks and 2

treatments (25 and 35 cm pre-grazing sward height). Eight 24-month-old Nellore

steers, 343 kg BW ± 7.40, cannulated in the rumen were used in this study.

4.2.2 Experimental treatments and management practices

Treatments consisted of 2 levels of energy supplementation (mineral

supplementation and 0.6% BW of ground corn as fed basis, combined with 2 pre-

grazing sward heights (25 and 35 cm), with a common stubble height of 15 cm for all

treatments. Pasture management was performed as a rotational grazing system, with

use of nitrogen fertilizer (40 kg/ha per grazing cycle). The targeted pre-grazing sward

height of 25 and 35 cm was based on research with marandu palisadegrass

correlating these measurements to 95 and 100% light interception (TRINDADE,

2007).

The experiment lasted 64 d and consisted of four experimental periods of 16 d

each. Animals were weighed (full BW) prior to the start of each experimental period

and the allocation of supplement was fixed for that experimental period. The first 8 d

88

were used to adapt the animals to the experimental treatments. The following 8 d

consisted of sample collections in the following order: on days 9 to 13 the fecal

samples collection was performed, on day 14 measurements of animal behavior were

taken, on day 15 collections of blood and urine samples were performed and on day

16 rumen fluid samples were collected. Animals had free access to water and

minerals supplement in every paddock. The supplemented and the no supplemented

experimental animals were brought daily to a management center close to the

experimental area where the supplement was in individual feed bunks from 1000h to

1040h. The no supplemented steers were also contained in the feed bunks. This

management center was also utilized for infusions of external marker and samplings.

4.2.3 Feed Samples Collection, Chemical Analysis and Calculations

Forage mass and morphological composition were determined in every

grazing cycle, before the steers entered every paddock and after they left. In each

evaluation, 3 randomized points were selected in the paddock, and in each point, the

material within a rectangle frame (0.5 m2) was cut to ground level. Total forage mass

was weighted, and a representative subsample of 0.5 kg was taken and separated

into leaf blades, stems (including leaves sheaths), and dead material (as indicated by

more than 50% of the tissue area being senescent) to determine the sward

morphological composition. After collection all samples were dried at 55º C. Pre and

post-grazing sward height, forage mass, and morphological composition are

presented in Table 6.

For chemical composition analysis pasture samples (grazing horizon 15 to 25

and 15 to 35 cm) were cut and collected using a rectangle frame (0.5 m2), from 3

distinct points prior to animals’ entry in every paddocks for every grazing cycle. Every

batch of ground corn was sampled. Feed samples were oven dried at 55oC for 72

hours and ground through a 1 mm screen (Wiley mill). Pasture samples were bulked

as one composite sample by treatment. Corn samples were bulked as one composite

sample.

Residual moisture content was determined by drying samples at 105ºC for 24

h. Organic matter content of samples was determined after incineration at 550ºC for 8

h in a muffle furnace (AOAC, 1990). Nitrogen content was determined by the Dumas

combustion method (Leco 2000, Leco Instruments Inc) and a conversion factor of

6.25 was used to convert total nitrogen to CP. Contents of NDF, ADF and lignin were

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determined according to Van Soest et al. (1991) with no sulphite for all samples

analyses and the addition of amylase for supplement analyses only. The NDIN and

ADIN were determined using micro Kjeldahl (AOAC, 1990) after digestion of samples

with neutral (no sulphite) and acid detergents (VAN SOEST et al., 1991). The TDN

values of feed samples were calculated according to Weiss et al. (1992). Forage

sample were also analyzed for soluble nitrogen (KRISHNAMOORTY et al, 1982) and

for non-protein nitrogen (NPN) (LICITRA et al, 1996) to estimate the protein fractions

according to CNCPS (SNIFFEN et al, 1992).

The data regarding pastures chemical and morphological composition are

presented in the companion article.


Samples of rumen fluid were collected to determine N-NH3 and VFA

concentrations. On day 16 of every experimental period rumen fluid samples

(approximately 100 mL) were collected at 0, 2, 4, 6, and 8 h after supplement feeding

with the use of a sampling probe (DANÉS et al., 2013). The rumen pH was measured

immediately and samples were frozen at -20°C for further analyses. Rumen fluid

samples were thawed at room temperature centrifuged (15,000 g, 4ºC, 30 min) and

analyzed for VFA using gas chromatography (PALMIQUIST; CONRAD, 1971), and

for N-NH3 by phenol-hypochlorite procedure (CHANEY; MARBACH, 1962).

Blood samples were collected on day 15 of the experimental period 4 h after

feeding. Blood was collected from the coccygeal vessels of each steer directly into a

lithium heparin coated vacutainer [Vacuette, Greiner Bio-One (Americana, SP,

Brazil)].

The vacutainers were inverted six to eight times and placed on ice. Plasma

was collected after centrifugation (3,000 g, 4º C, 20 min) and stored frozen at -20°C

for further analyses. Levels of plasma glucose were analyzed by automatic

biochemical analyzer YSI 2700 Select.

4.2.5 Forage Intake and digestibility

Forage intake was calculated from total fecal excretion and the digestibility of

feeds. On day 1 and for 13 consecutive days, chromium oxide (Cr2O3), previously

weighed in two paper capsules (5 g each), was dosed into the rumen to all steers,

90

one dose right after supplement feeding in the morning (1000 h) and a second dose

in the afternoon (1700 h). Fecal grab samples (approximately 200 g per animal) were

collected between days 9 to 13, twice a day (1000 h and 1700 h), dried at 55°C for

72 h and ground through a 1-mm screen (Wiley mill). Equal amounts of fecal DM

were mixed to obtain one representative sample for each steer for every

experimental period. The digestibility of feeds (pasture and supplement) was

estimated using the indigestible neutral detergent fiber (iNDF) content as an internal

marker (CASALI et al., 2008). The CP and NDF concentrations were analyzed in the

feces and multiplied by fecal production, to calculate CP and NDF digestibility.

For analyses of fecal chromium concentration, approximately 0.3 g of sample

was digested with 6 mL nitric acid and 2 mL perchloric acid and then made up to 30

mL with RO water (VEGA; POPPI, 1997). The digested samples were analyzed using

an inductively coupled plasma spectrometer (ICP-OES).

Forage intake was calculated dividing forage fecal excretion (total fecal excretion

minus the amount resultant from the supplement) by the forage indigestibility.

4.2.6 Animal Behavior

Grazing behavior measurements took place on d 14 of each period. Four

trained observers were assigned to visually determine grazing (eating plus

searching), rumination, and idling time every 5 min for consecutive 24 hours The

number of observations for each activity during a 24 hours period was multiplied by 5

minutes to get the daily amount of time spent in that activity. While grazing, cattle

search, acquire into the mouth, masticate, and swallow herbage (Gibb, 1998). The

observers also counted bite rate (bites/min) using the time spent to 20 bites

(PENNING; RUTTER, 2004).

Each eating step was considered a feeding station (RUYLE; DWYER, 1985;

ROOK et al., 2004). The feeding stations per minute were calculated considering the

time spent to 10 feeding station divided by 10. Also, it was measured the number of

steps between feeding stations, and then it was calculated the total steps per day,

multiplying the steps per minute by grazing time (min).

91


Nitrogen intake was calculated by the sum of intakes of N from forage and

supplement origins. Apparent efficiency of N use (assuming no retention or

mobilization of N in the body) was calculated for every steer by dividing mean

retained N (N intake minus N in urine and feces) by the average N intake.

Microbial synthesis was estimated through the excretion of purine derivatives

(STANGASSINGER et al., 1995). Spot urine samples were collected on day 15 of

each experimental period, in a single sample collected approximately 4 h after

supplement feeding. Sub-samples of urine (10mL) were acidified (pH<3.0) using 40

mL of 0.036 N H2SO4 and stored at −20°C (VALADARES et al., 1999). Urine samples

were analyzed for creatinine, allantoin and uric acid using a HPLC (PIMPA;

BALCELLS, 2002), and for N using micro Kjeldhal (AOAC, 1990).

Total urine production was estimated using the creatinine content in the

sample, assuming that excretion of creatinine is constant (0.213 mmol kg BW -1)

(CHIZZOTTI et al. 2008). The excretion of purine derivatives (PD) was then

calculated as the sum of allantoin and uric acid excreted in urine, expressed in

mmol/d. The analyses of PD were performed according to Pimpa et al. (2001) using

values of endogenous contribution of PD for zebu cattle, and the intestinal flow of

microbial nitrogen compounds was calculated on the basis of microbial purines

absorbed as described by Chen e Gomes (1992).


The in vitro rumen fermentation analysis were described and discussed in the

companion article because the incubated feed samples were the same for both

experiments.

4.2.9 Statistical Analysis

The experimental design was a double 4x4 Latin Square design, with eight

animals, four treatments and four periods. All variables, except the rumen

fermentation kinetics and its predicted parameters, were analyzed using the PROC

MIXED package of SAS statistical system (SAS® version 9.2©, 2008). The rumen

fermentation kinetics variables were analyzed in a completely randomized design

using PROC GLM package of SAS statistical system (SAS® version 9.2©, 2008). The

92

model included treatment, animal and period as sources of variation to analyze the

data of forage, nutrient intake, animal behavior, N excretion, blood parameters and

microbial synthesis. Treatments and periods were considered fixed, while animal and

overall errors were considered random effects. The model used to analyze rumen

parameters (pH and concentrations of VFA and N-NH3) collected at different times

after feeding, included treatment, period, animal, hours post-feeding as repeated

measures and treatment x hours post-feeding interaction as sources of variation.

All variables were considered fixed, except animal, whole-plot error, and

subplot error, which were considered random. The covariance structure used for

repeated measures analyses was chosen based on Bayesian Information Criteria

parameter of SAS statistical system (SAS® version 9.2©, 2008) closer to zero. The

averages were calculated by Least squares means and adjusted for comparison by

Tukey test at 5% probability. The averages were calculated by Least squares means

and adjusted for comparison by Tukey test at 5% and 10% probability. Thus,

significance was declared at P<0.05 and P<0.01.

The parameters of rumen fermentation kinetics predicted by the equations

were fitted by nonlinear models using PROC NLIN of the SAS statistical system

(SAS® version 9.2©, 2008). Necessarily, all data sets were tested before the final

overall analysis to ensure that all the assumptions of analysis of variance (additive

model, independence of errors, data normality and homoscedasticity) were met.

4.3 Results

4.3.1 Pasture and Supplement Samples

The data regarding pastures managements were presented in the companion

article.

4.3.2 Forage Intake and Animal Behavior

There were interactions between grazing management and energy

supplementation (Table 15) for grazing time (P<0.05), for rumination time (P<0.1), for

steps between feeding stations (P<0.1) and for steps per days (P<0.10)

When 0.6% BW of corn was fed for animals grazing pastures managed with

25 cm of sward height, grazing time was reduced in 22 minutes (P<0.05), but for

93

pastures managed with 35 cm, grazing time was reduced 120 minutes (P<0.05)

(Figure 1).

However, this reduction on steps per day and steps between feeding stations

were more intense in animals maintained in pastures managed at 35 cm of pre-

grazing sward height (Figure 2 and Figure 3), as shown by the significant (P<0.10)

interaction between grazing management and energy supplementation presented in

the Table 15.

Animals grazing pastures managed with 25 cm sward height decreased the

ruminantion time (P<0.05) to 400 min/d for 372 min/d, when 0.6% BW as fed.

However, when the 0.6% BW was fed for animals grazing pastures managed with 35

cm sward height there was no difference (P>0.05) in ruminantion time in comparison

with non-supplemented animals (488 and 496 min/d for 0 and 0.6% BW,

respectively).

Animals kept on pastures managed with 25 cm, presented more time resting

and greater bite rates (P<0.05) compared to cattle grazing pasture with 35 cm sward

height as the start grazing point (Table 15).

The number of feeding stations per minute was greater (P<0.10) for animals

grazing pastures with 25 cm. The steps per minutes were not affected by grazing

manegement (P>0.05). However, as consequence of greater grazing time for 35 cm

treatment the total steps per day also was greater (P<0.05) for 35 cm pre-grazing

sward height (Table 15).

The energy supplementation had no effect (P>0.05) on resting time, bite rates

and feeding stations but it increased (P<0.05) steps per minute.

94


grazing behavior

Management, cm

Level of supplementation,

% BW P value SEM

25 35 0 0.6 M S M*S

Grazing time, min/d 390.05 434.61 447.80 376.80 0.0251 0.0011 0.0152 29.23 Ruminanting time, min/d 392.06 441.66 401.66 431.97 0.2416 0.4687 0.0870 36.43 Rest time, min/d 607.72 502.28 543.49 566.51 0.0143 0.5011 0.6450 35.05 Bit rate, bites/min 29.72 19.59 23.41 25.90 0.0360 0.5980 0.5025 3.69 Feeding stations/min 5.17 4.12 4.75 4.55 0.0615 0.7112 0.4503 0.56 Steps between FS 1.14 1.62 1.54 1.22 0.0002 0.0082 0.0623 0.10

Steps/min 5.96 6.52 6.97 5.50 0.3580 0.0253 0.6137 0.64

Steps/day 2244.1 2912.7 3089.4 2067.8 0.0186 0.0010 0.0791 266.6 1Standard error of the means, M=management, S=supplementation, M*S=interactions between

management and supplementation.

Figure 1 - Interactions between grazing management and energy supplementation on grazing time

95

2,518

1,971

3,661

2,164

0

500

1000

1500

2000

2500

3000

3500

4000

4500

0 0.6% of BW 0 0.6% of BW

25 cm 35 cm

Ste

ps/

day

Figura 2 - Interactions between grazing management and energy supplementation on steps per day

1.191.09

1.88

1.36

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

0 0.6% of BW 0 0.6% of BW

25 cm 35 cm

Ste

ps

be

twe

en

fe

ed

ing

stat

ion

s

Figura 3 - Interactions between grazing management and energy supplementation on steps between feeding stations

There was no interaction between grazing managements and level of energy

supplementation (P>0.05) for feed intake. Cattle grazing pastures with 25 cm

(correlated to 95% LI) had greater intakes of forage, of total DM, of digestible DM, of

NDF and of CP (P<0.05) than animals grazing pastures managed with 35 cm as start

grazing point (Table 16). Energy supplementation reduced forage and NDF intakes

(P<0.05), with no effect on CP intake (P>0.05) (Table 16). The total DM and energy

intake were increased when 0.6% BW of corn was fed. The substitution rate for

feeding 0.6% BW of ground corn was 0.72 (P<0.05).

96

Table 16 - Effects of energy supplementation and grazing management on forage,

total, digestible, NDF and CP intakes and substitution rate

Management,

cm


% BW P value

SEM

25cm 35cm 0 0.6 M S M*S

Forage DMI, % BW 1.88 1.22 1.77 1.33 0.0032 0.0385 0.7159 0.31

Total DMI, % BW 2.18 1.52 1.77 1.93 0.0030 0.0430 0.7155 0.33 Digestible DMI, % BW

1.47 0.99 1.05 1.36 0.0013 0.0251 0.3416 0.17

NDF DMI, % BW 1.14 0.81 1.07 0.87 0.0163 0.0961 0.6350 0.20

CP DMI, % BW 0.29 0.16 0.22 0.22 0.0001 0.9234 0.9432 0.04

Substituion rate 0.00 0.00 0.00 0.72 0.4734 0.0030 0.4921 0.27

Forage DMI, kg/d 5.40 3.60 5.19 3.81 0.0034 0.0188 0.7027 0.77

Total DMI, kg/d 6.31 4.51 5.20 5.62 0.0027 0.0438 0.6914 0.74

Digestible DMI, kg/d 4.28 2.95 3.24 3.99 0.0011 0.0452 0.5964 0.41

NDF DMI, kg/d 3.27 2.38 3.15 2.50 0.0118 0.0546 0.6155 0.46

CP DMI, kg/d 0.83 0.48 0.66 0.66 0.0001 0.9965 0.9438 0.10 1Standard error of the means,

2= kg of reduction on forage DMI for kg of supplemented fed,

M=management, S=supplementation, M*S=interactions between management and supplementation


The forage DM and the forage and diet CP digestibility were greater (P<0.05)

for 25 cm pre-grazing sward height than for 35 cm, while forage NDF, diet NDF and

diet DM were not affected (P>0.05) by pasture management (Table 17). Energy

supplementation increased forage NDF digestibility (P<0.05) and decreased (P<0.05)

forage CP digestibility, with no negative effects (P>0.05) on forage DM digestibility.

When corn was fed at 0.6% the diet DM digestibility was increased (P<0.05) as well

as the diet NDF digestibility (P<0.10), with no effects on CP digestibility (Table 17).

97

Table 17 - Effects of energy supplementation and grazing management on forage

and diet nutrients digestibility

Management,

cm


% BW P value

SEM

25cm 35cm 0 0.6 M S M*S

Forage digestibility (%) DM 62.94 57.64 61.70 58.90 0.0001 0.1530 0.1652 1.44

CP 71.41 58.71 69.91 60.21 0.0001 0.0005 0.1311 2.39

NDF 65.15 64.36 62.71 66.8 0.5756 0.0077 0.7914 1.27

Diet digestibility (%) DM 68.03 65.74 61.83 71.94 0.2752 0.0001 0.8657 1.77

CP 74.53 66.33 70.03 70.83 0.0017 0.7331 0.7191 2.16

NDF 64.48 63.39 62.68 65.19 0.4408 0.0823 0.9495 1.19 1Standard error of the means, M=management, S=supplementation, M*S=interactions between


4.3.4 Rumen Fluid and Plasma Glucose

There was no interaction between grazing management and energy

supplementation (P>0.05) for rumen pH, VFA, and plasma glucose (Table 18).

However, there was interaction between energy supplementation and grazing

management for N- N-NH3. The rume N-NH3 was reduced more intense feeding

0.6% BW of energy supplementation for animals grazing pastures managed with 35

cm sward height (Figure 4).

The rumen pH was less (P<0.1) for cattle grazing the 25 cm sward height and

for cattle supplemented with ground corn at 0.6% BW (P<0.05). The rumen N-NH3

was greater (P<0.05) for cattle grazing the 25 cm pasture and less (P<0.05) for cattle

supplemented with ground corn at 0.6% BW. Total VFA concentration was not

affected (P>0.05) by treatments, but supplementing energy increased rumen

propionate proportion (P<0.05) and decreased rumen acetate and acetate:propionate

(A:P) ratio (P<0.05). Plasma glucose was not affected by increasing energy

supplementation and grazing management (P>0.05).

98

Table 18 - Effects of energy supplementation and grazing management on rumen

pH, NH3/N, volatile fatty acids (VFA) and plasma glucose

Management, cm


% BW P value SEM

25 35 0 0.6 M S M*S

pH 6.39 6.46 6.50 6.35 0.0854 0.0001 0.7149 0.14

N-NH3,mg/dL 12.00 10.01 12.62 9.38 0.0061 0.0001 0.0682 1.19

Total VFA,mM 142.83 135.14 135.13 142.84 0.3597 0.3585 0.3615 11.88

mol/100 mol

Acetate 68.85 68.83 70.69 66.99 0.9854 0.0001 0.3378 0.78 Propionate 20.89 20.50 18.90 22.48 0.6214 0.0001 0.1455 0.81 Butyrate 10.61 11.17 11.10 10.67 0.2556 0.3858 0.6490 0.35 A:P 3.48 3.64 3.94 3.18 0.2681 0.0001 0.1915 0.14 Plasma glucose, mg/dL 55.34 54.87 56.13 55.83 0.8723 0.7634 0.3185 1.76



13.0

11.112.3

7.9

0

2

4

6

8

10

12

14

16

0% 0.60% 0% 0.60%

25 cm 35 cm

N-N

H3

(m

g/d

L)

Figure 4 - Interactions between grazing management and energy supplementation on N-NH3


There was no interaction (P>0.05) between energy supplementation and

grazing management for nitrogen use and microbial synthesis (Table 19). The energy

supplementation increased (P<0.05) the nitrogen retention, efficiency of N use and

microbial protein synthesis of cattle grazing fertilized Palisadegrass pasture.

Managing the pasture with 25 cm pre-grazing sward height caused a huge increase

99

(P<0.05) in the intake of N with greater retention of N (P<0.05) and greater efficiency

of N utilization (P<0.05) by cattle grazing fertilized tropical pasture.

Table 19 - Effects of energy supplementation and grazing management on nitrogen

intake, nitrogen urine, nitrogen feces, nitrogen retained (% N intake),

microbial protein (MicP) and microbial efficiency (MicEf)

Management,

cm

Level of

supplementation,

% BW

P value SEM1

25 35 0 0.6 M S M*S

N fecal2 39.94 36.56 38.12 38.38 0.6142 0.9688 0.9688 7.24

N urinary2 41.51 33.22 40.73 34.00 0.0205 0.0553 0.2951 2.43

N excretion2 80.93 69.78 78.56 72.14 0.1202 0.3575 0.4936 7.76

N intake2 135.03 98.68 112.77 120.95 0.0129 0.5442 0.9895 14.37

N retention2 53.94 29.94 35.15 48.73 0.0205 0.1646 0.5826 8.65

N retention,

%N intake) 39.80 30.32 30.16 39.28 0.0463 0.0477 0.8794 3.10

MicP2 401.59 376.83 357.07 421.35 0.3842 0.031 0.4118 24.47

MicEf.

g/kg of TDN 108.22 134.80 124.41 118.61 0.0941 0.7026 0.5387 16.38


2g/day, M=management, S=supplementation, M*S=interactions

between management and supplementation

4.4 Discussion

4.4.1 Animal Behavior and Forage Intake

As discussed in the companion article, the high fiber content and low particle

fragility of tropical forages are the main factors that promote rumen fill effect, being

responsible to limit forage intake in grazing animals (STOBBS, 1973; POPPI et al.,

1980, 1981a, 1981b, 1985; ALLEN, 200). However, the sward structure also imposes

limitations on forage intake, as demonstrated in this study.

The effect of grazing management on DMI was the same discussed in the

companion article, where the adoption of a start grazing point based on a sward

height (25cm) correlated with 95% light interception for marandu Palisadegrass

promoted increases on DMI (total, forage, digestible, NDF and CP) as response to

100

decreases on harvesting efficiency (Table 15). The greater proportions of stems and

senescente material in the 35 cm pasture contributed to reduce harvest efficiency

during the grazing down process acting as a horizontal barrier (UNGAR et al., 2001;

BENVENUTTI et al., 2006, 2009; DRESCHER et al., 2006; BAGGIO et al., 2009).

In the companion article were observed reductions in bite rate of 34% when

animals were managed in pastures with 35 cm in comparison with 25 cm of initial

sward height. The same value (34%) was observed in the present study, indicating a

consistent impact of sward height and structure on grazing behavior (CARVALHO

1997; BAGGIO et al., 2009).

In the current study was aimed to increase the amount of ground corn

supplementation (0.6% BW) to evaluate its interaction with grazing management.

Thus, when the level of corn was 0.6% BW, there was interaction between grazing

management and energy supplementation for grazing time.

In both grazing management the grazing time was reduced when cattle were

supplemented. However, for the 25 cm pastures animals fed corn at 0.6% BW grazed

22 minutes less while for the 35 cm pastures, feeding corn decreased grazing time in

120 minutes (Figure 1). This fact indicates that in pastures where the sward

strucuture is not adequate, the supplementation may cause greater substitution rate

than in pastures with more favorable sward structure. This may limit the increment in

energy intake caused by supplementation in not well managed pastures.

The energy supplementation affected the number of feeding stations per

minute (P<0.05), where supplemented animals had less feeding stations per minute

than non-supplemented animals. It means that the supplemented animal spent more

time per feeding station, what suggests that supplemented animals may select more

and use better each feeding station (Table 15). Non-supplemented animals need to

harvest more due to energy status and it reflecst on the number of feeding stations.

According to several authors (THOMSON et al., 1985; GREGORINI et al., 2007,

2009) the level of hunger influences forage intake rate and grazing dynamics.

The combination between greater bite rates and less grazing time suggests

greater harvesting efficiency, what may be confirmed by greater DMI (Table 16) for

animals grazing pastures with 25 cm of initial sward height. Also, this information

suggests less energy expenditure for grazing activities, for the 25 cm treatments,

supported by all mentioned parameters, mainly by greater resting time.

101

Animals grazing pastures managed at 35 cm spent more time per feeding

station (Table 15), suggesting that the greater forage mass and the time to

manipulating the forage before to eat, may have influenced the time spent per

feeding station (BAGGIO et al., 2009). The number of steps between feeding stations

and steps per day found for 35 cm treatments showed that animals walked more

searching for better feeding stations. The forage allowance at each feed station

determines the time spent by the animal at the feeding station (BAGGIO et al., 2009).

This result corroborates with some authors that found effects of

supplementation on displacement patterns, where non-supplemented beef heifers

traveled greater distances and visited more feeding stations per minute, while the

opposite happened for supplemented animals (GLIENKE et al., 2010). A hypothesis

raised was that supplemented animals spent less time searching for feeding stations

because they had their nutritional requirements met. In the other hand, other study

reported that supplemented animals were more selective and covered greater

distances in search for better feeding stations (ADAMS, 1985).

The interaction between grazing managements and energy supplementation

for steps between feeding stations and steps per day is showed in the Figure 2 and

Figure 3. In both interactions is possible to observe that the impact of energy

supplementation was greater in animals maintained in pastures managed at 35 cm of

sward height, corroborating whit the reported for grazing time. Even without detect

interactions for forage DMI and substitution rate, the behavioral parameters suggest

that the substitution rate could have been greater for supplemented animals grazing

pastures managed with 35 cm as the start grazing point.

Feeding ground corn at 0.6% BW affected forage DMI, total and digestible DM

for both 25 and 35 cm grazing managements (Table 16). The interaction observed for

grazing time was not observed for forage intake.

Energy supplementation decreased forage DMI as consequence of

substitution effect, but it increased total and digestible DMI (Table 16), differently of

the observed in the companion article, where 0.3% BW was not able to increase

digestible DM. Dórea (2011) also reported that feeding ground corn at 0.3% BW for

cattle on tropical pastures under rotational grazing failed to increase energy intake.

Correia (2006), fed 0, 0.3, 0.6 and 0.9% BW of an energy supplement for cattle on a

high stocked pasture of Palisade grass under rotational grazing. The 0.3% level had

102

slight effect on cattle ADG and a great impact on pasture stocking rate compared to

the no supplemented treatment.

Feeding 0.6% BW or greate was very effective to increase ADG and beef

production per area. Costa (2007) and Dell Agostinho Neto (2010) also reported that

feeding ground corn at 0.6% BW was a confident level to increase cattle

performance, pasture stocking rate and beef production per area.


The energy supplementation did not increase the total VFA as supported by

other researches with readily fermentable fiber and starch sources (MARTIN;

HIBBERD, 1990; CAREY et al., 1993). However the molar proportions of acetate was

lower (P<0.05) in supplemented steers than in non-supplemented steers.

Conversely, molar proportions of propionate were greater (P<0.05) for supplemented

steers than for no supplemented steers (Table 6). As a result, the acetate:propionate

was lower for supplemented steers. Thus, cattle supplemented with corn (0.6%BW)

even without an increase in total VFA concentration had more propionate available

for metabolism than no supplemented cattle (Table 18). Increased propionate and

decreased acetate as a result of concentrate inclusion in the diet is well documented

(HORN; McCOLLUM, 1987). Butyrate molar proportions did not change for any

treatments. Butyrate proportions have been documented to increase with increasing

starch consumption (STERN et al., 1978; GRIGSBY et al., 1991).

The greater intake of higher quality forage of cattle on the 25 cm pastures

resulted in lower ruminal pH (6.39 vs 6.42) compared to the 35 cm pastures. Feeding

0.6% of ground corn also decreased rumen pH (6.35 vs 6.50) compared to no

supplementation. However even for supplemented cattle rumen pH remained always

above 6.2 (Table 18). According to Mertens (1977) the forage fiber digestion declines

when ruminal pH fell below 6.7. Later, Ørskov (1982) and Mould et al. (1983)

indicated that ruminal pH below 6.2 would reduce the activity of cellulolytic bacteria

and digestion of straw, respectively. These researchers indicated that depressions in

ruminal pH could be responsible for reductions in forage fiber digestibility associated

with grain supplementation. Some studies indicated that populations of cellulolytic

bacteria were reduced in pH ranges from 5.7 to 6.2, whereas soluble carbohydrate

fermenting bacteria persist until ruminal pH ranges from 4.6 to 4.9 (RUSSELL et al.,

1979; RUSSELL; DOMBROWSKI, 1980) Sensitivity of ruminal bacteria to pH and

103

shifting in the bacterial populations in response to reduced pH have been suggested

as reasons for reduced forage digestion and consequent reduced forage intake by

grazing ruminants fed high energy supplements (HORN; McCOLLUM, 1987).

However, the results in this work are corroborated with some reports published in the

literature (CATON; DHUYVETTER, 1997; SANTOS et al., 2009) where is

emphasized that there is lack of reliable data pointing to rumen pH as the cause of

reductions on fiber digestibility, and attributed effects on DMI to other factors.

As found in the companion article the N-NH3 in the rumen fluid of steers was

reduced in two different ways: a) due to energy supplementation (0.6% BW) (12.62

vs 9.38 mg/dL for 0 and 0.6% BW of energy supplementation, respectively); b) due to

grazing management (12.00 vs 10.01 mg/dL for 25 and 35 cm pre-grazing sward

height, respectively) (Table 18).

However, due to lower CP content and protein fractions characteristcs for

pastures managed with 35 cm sward height, when 0.6% of energy supplementation

was fed, the N-NH3 in the rumen fluid was reduced more intense, in comparison with

animals maintained in pastures managed with 25 cm sward height. Its suggests that

even with lower levels of N-NH3 in non-suppmenebted animals kept in 35 cm grazing

management, the utilization of this N-NH3 by rumen microorganism were greater than

in animals grazing pastures with 25 cm sward height, resulting in the more intense N-

NH3 reduction observed.

The greater intake of readily fermentable carbohydrates may explain the

supplementation effect, what provided more substrate for microbial synthesis

(RUSSELL; MARTIN, 1984) as can be observed in Table 18. According to Grigsby et

al. (1991), the reduction of ruminal N-NH3 in steers supplemented with soybean hulls

was attributed to increased utilization of N-NH3for microbial protein synthesis. It is

reported in the literature that microbial synthesis is increased when starch and

proteins degradation are synchronized (RUSSELL et al., 1992; FIRKINS, 1996;

BACH et al., 2005).

As reported in the companion article, the pastures managed at 25 cm of sward

height contained greater CP content and also had A+ B1 fraction 28% higher than

pastures managed at 35 cm pre-grazing sward height, what may explain the highest

N-NH3 concentrations in pastures managed at 25 cm sward height.

There was no effect of any treatments on blood glucose (P>0.05), what is

corroborated by data from Reed et al. (2007) and by Hersom et al. (2004).

104


Feeding ground corn at 0.6% BW improved the utilization of dietary N (Table

19). This improvement is explained by more rapidly fermentable carbohydrate in the

rumen, what reduced rumen N-NH3, and increased microbial synthesis (RUSSELL et

al., 1992; OWENS; GOETSCH, 1993), and also because of more of the absorbed N

is incorporated in animal tissues. Animals fed corn at 0.3% BW did not reduce N in

urine neither increased N retention, as showed in companion article and reported by

Dorea (2011), because energy intake was not increased. For cattle grazing tropical

forages with 5.08, 7.55 and 5.16 % CP (LAZZARINI et al., 2009; FIGUEIRAS et al.,

2010; SOUZA et al., 2010) it was not observed improvements on microbial synthesis

when protein supplements were fed, suggesting that energy was limiting the

increases on microbial growth.

Improving pasture management was an effective way to increase protein

intake, due to greater CP content of forage and greater forage intake. Even so, cattle

grazing the 25 cm pastures succeeded to excret less N in the urine and to retain

more N than cattle grazing the 35 cm pastures, because of greater energy intake.

The average estimate of microbial efficiency in the current study was 121.5 g

microbial CP/ kg TDN. This value is close to the average found in the literature (120.5

g microbial CP/kg TDN) by Lazzarini et al. (2009), Figueiras et al. (2010), Souza et

al. (2010). The average found in the literature as well as in this current study is close

with suggested by Valadares Filho et al. (2010), but is a little lower than suggested by

the NRC (1996) (Table 19).


Curiously the energy supplementation did not increase forage DM digestibility,

but increased the NDF digestibility (Table 17). The forage CP digestibility was

decreased with the energy supplementation and this fact can be explained by the

increase in microbial synthesis. This results in less N being excreted via urine and

more N excreted in feces.

Twelve articles published in the literature (MARTIN; HIBBERD, 1990; CAREY

et al., 1993; FAULKNER et al., 1994; ELIZALDE et al., 1998; DETMAN et al., 2001;

LARDY et al., 2004; RICHARDS et al., 2006; PAVAN; DUCKET, 2008; SALES et al.,

2008a, 2008b; NASCIMENTO et al., 2010, 2009) were compiled and a meta-analysis

105

was run to test the effect of concentrate supplementation on NDF digestion on total

tract. There was not effect on NDF digestion, unlike reported on a few papers

published in the literature (HANNAH et al., 1989; ZORRILLA-RIOS et al., 1989;

VANZANT et al., 1990; PORDOMINGO et al., 1991).

The NDF digestibility in the total tract found in this present study (65.19%)

(Table 17) is close to observed in the meta-analysis intercept (64.00%). This founds

corroborate with other several studies (LAKE et al., 1974; KRYSL et al., 1989;

BRANINE; GALYEAN, 1995; CATON; DHUYVETTER, 1997; SANTOS et al., 2009;),

which reported improvements or no effect of energy supplementation on DM, OM or

NDF digestibility for cattle consuming forage diets.

Pastures managed with 25 cm produced forage with greatest DM digestibility.

The major difference in chemical composition between the 25 and the 35 cm

pastures was the content of NDF. However, for both pastures, NDF digestibility was

greater than DM digestibility and the digestibility NDF was not different between the

pastures. Lopes (2011) reported that FDN from well managed tropical grasses

presented greater in vitro digestibility than alfalfa silage.

4.5 Conclusions

For cattle grazing Brachiaria Braizantha cv. Marandu, the adoption of 25 cm of

sward height as the start grazing point is an effective tool to increase energy intake

and decrease energy expenditure with grazing activity.

Feeding ground corn at 0.6% BW is an effective tool to increase energy intake

and to decrease energy expenditure with grazing activity.

Substituion of grain for forage may be greater when pastures are not well

managed in rotational grazing systems.

Improving pasture management and feeding corn at 0.6% BW are effective

tools to improve forage CP utilization.

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